So this is how things _seem_ to go for this Bug:
"""
(gdb) bt
#0  0xa62c5649 in dbi_shutdown_r () from /usr/lib/i386-linux-gnu/libdbi.so.1
#1  0xa62c56dd in dbi_shutdown () from /usr/lib/i386-linux-gnu/libdbi.so.1
#2  0xa62da2f4 in gnc_module_finalize_backend_dbi () from /usr/lib/i386-linux-gnu/gnucash/gnucash/libgncmod-backend-dbi.so
#3  0xa62da314 in qof_backend_module_finalize () from /usr/lib/i386-linux-gnu/gnucash/gnucash/libgncmod-backend-dbi.so
#4  0xb7c9fb79 in qof_finalize_backend_libraries () from /usr/lib/i386-linux-gnu/gnucash/libgnc-qof.so.1
"""

- In frame #4 some Gnucash library function is called without any arguments:
"""
253 void
254 qof_finalize_backend_libraries(void)
255 {
256     GSList* node;
257     GModule* backend;
258     void (*module_finalize_func) (void);
259 
260     for (node = backend_module_list; node != NULL; node = node->next)
261     {
262         backend = (GModule*)node->data;
263 
264         if (g_module_symbol(backend, "qof_backend_module_finalize",
265                             (gpointer)&module_finalize_func))
266             module_finalize_func();
267 
268     }
269 }
"""

- This seems to call a formerly registered function by the name of "qof_backend_module_finalize", if it exists, which it must because it is found in the conditional, for each and one of the backends (dbi being one, don't know if there are others though):
"""
1939 qof_backend_module_finalize( void )
1940 {
1941     gnc_module_finalize_backend_dbi();
1942 }
1943 #endif /* GNC_NO_LOADABLE_MODULES */
1944 
1945 void
1946 gnc_module_finalize_backend_dbi( void )
1947 {
1948     dbi_shutdown();
1949 }
"""

- That lead us to frames #3 and #2 without anything very interesting to say about it. Let's dig deeper and closer to the real deal:
"""
 274 void dbi_shutdown() {
 275     dbi_shutdown_r(dbi_inst_legacy);
 276 }
"""

- So we are calling some function which description in the library documentation is as follows:
"""
Frees all loaded drivers and terminates the libdbi instance. You should close each connection you opened before shutting down, but libdbi will clean up after you if you don't.
Arguments
Inst: The instance handle
"""

- Then control passes on to frame #0, and then die after a SIGSEGV:
"""
 243 void dbi_shutdown_r(dbi_inst Inst) {
 244     dbi_inst_t *inst = (dbi_inst_t*) Inst;
 245     dbi_conn_t *curconn = inst->rootconn;
 246     dbi_conn_t *nextconn;
 247     
 248     dbi_driver_t *curdriver = inst->rootdriver;
 249     dbi_driver_t *nextdriver;
 250     
 251     while (curconn) {
 252         nextconn = curconn->next;
 253         dbi_conn_close((dbi_conn)curconn);
 254         curconn = nextconn;
 255     }
 256     
 257     while (curdriver) {
 258         nextdriver = curdriver->next;
 259         curdriver->functions->finalize(curdriver);      
 260         _safe_dlclose(curdriver);
 261         free(curdriver->functions);
 262         _free_custom_functions(curdriver);
 263         _free_caps(curdriver->caps);
 264         free(curdriver->filename);
 265         free(curdriver);
 266         curdriver = nextdriver;
 267     }
 268 #if HAVE_LTDL_H
 269         (void)lt_dlexit();
 270 #endif  
 271     free(inst);
 272 }
"""

- As I haven't compiled gnucash from scratch with debug information, I can only theorize, but considering libdbi must be some quite frequently used piece of code, I can't see how the library code can be broken

- I rather believe gnucash is blindly calling the dbi_shutdown without making sure there is really any dbi connection to shut down in the first place. Therefore the "dbi_inst_legacy" may be NULL or garbage, and as soon as it tries to be dereferenced (see line 244 above), the code crashes.

- So probably the problem is with gnucash logic, which tries to shut down a dbi connection because it has registered the driver during initialization, but without really checking on shut down if there was a connection open (or still open)

- In the end, the fix may be as easy as adding some check to "gnc_module_finalize_backend_dbi" to see if there is really any connection to shut down, to only call " dbi_shutdown" when really necessary. Being a global, checking for "dbi_inst_legacy" being NULL is probably enough, lacking any better way of checking for any outstanding DBI connection

All in all, a 15 minute investigation for me and a 5 minute fix for any gnucash package or upstream maintainer.

Hope this gets looked into ASAP to have a working application without it crashing every time on shut down.