The pointer parameter value of the free function may be NULL

[xiaodonghu]

The function sprint_realloc_objid may return NULL in abnormal case.
So, the function fmtoid and fmtmsg_var may return NULL.

However, in these function log_results log_pdu_header and log_invalid_oid_trees, the tempory value var_str point to the return value of fmtmsg_var may be NULL.

In these function, snmp_log have checked whether var_str is NULL or not, but free doesn't check it.
When the var_str is NULL, free(NULL) will make the program crashed.

So we need to add the checking before free function.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/672935

[Ghada Khalil (gkhalil)]

Low priority / Not Gating - code cleanup suggestion / no noted system impact

[OpenStack Infra (hudson-openstack)]

Change abandoned by Bart Wensley (<email address hidden>) on branch: master
Review: https://review.opendev.org/672935
Reason: This patch has been idle for more than six months. I am abandoning it to keep the review queue sane. If you are still interested in working on this patch, please unabandon it and upload a new patchset.

[Ambarish Das (hiambar)]

As per C coding standard,compiler will treat free(NULL) as a no-op.(https://en.cppreference.com/w/c/memory/free)

This issue can be closed as invalid/won't fix.