Yeah, looking at the patch it's clear the NULL setting was accidentally (?) left off:

-        if (physPortID) {                                                                                                                                                                                                                                                    
-            if (firstEntryName) {                                                                                                                                                                                                                                            
-                /* we didn't match the provided phys_port_id, but this                                                                                                                                                                                                       
-                 * is probably because phys_port_id isn't implemented                                                                                                                                                                                                        
-                 * for this NIC driver, so just return the first                                                                                                                                                                                                             
-                 * (probably only) netname we found.                                                                                                                                                                                                                         
-                 */                                                                                                                                                                                                                                                          
-                *netname = firstEntryName;                                                                                                                                                                                                                                   
-                firstEntryName = NULL;                                                                                                                                                                                                                                       
-                ret = 0;                                                                                                                                                                                                                                                     
-            } else {                                                                                                                                                                                                                                                         
-                virReportError(VIR_ERR_INTERNAL_ERROR,                                                                                                                                                                                                                       
-                               _("Could not find network device with "                                                                                                                                                                                                       
-                                 "phys_port_id '%s' under PCI device at %s"),                                                                                                                                                                                                
-                               physPortID, device_link_sysfs_path);                                                                                                                                                                                                          
-            }                                                                                                                                                                                                                                                                
+        if (firstEntryName) {                                                                                                                                                                                                                                                
+            /* we didn't match the provided phys_port_id / find a                                                                                                                                                                                                            
+             * phys_port_name matching VIR_PF_PHYS_PORT_NAME_REGEX / find                                                                                                                                                                                                    
+             * as many net devices as the value of idx, but this is                                                                                                                                                                                                          
+             * probably because phys_port_id / phys_port_name isn't                                                                                                                                                                                                          
+             * implemented for this NIC driver, so just return the first                                                                                                                                                                                                     
+             * (probably only) netname we found.                                                                                                                                                                                                                             
+             */                                                                                                                                                                                                                                                              
+            *netname = firstEntryName;                                                                                                                                                                                                                                       
+            ret = 0;                                                                                                                                                                                                                                                         

either using g_steal_pointer() or just manually setting firstEntryName = NULL (as the code previously did) should fix things, since firstEntryName is automatically freed at the end of the function (since it's declared with g_autofree).