main/pbx: Don't attempt to destroy a previously destroyed exten/priority tuple

When a PBX registrar is unloaded, it will fail to remove its extension from
the context root_table if a dialplan application used by that extension is
still loaded. This can be the case for AGI, which can be unloaded after several
of the standard PBX providers. Often, this is harmless; however, if the
extension's priorities are removed during the failed unloading *and* the
dialplan application later unregisters, it leaves a ticking timebomb for the
next PBX provider that attempts to iterate over the extensions. When that
occurs, the peer_table pointer on the extension will already be set to NULL.
The current code does not check to see if the pointer is NULL before passing
it to a hashtab function this is not NULL tolerant.

Since it is possible for the peer_table to be NULL when we normally would not
expect that to be the case, the solution in this patch is to simply skip over
processing an extension's priorities if peer_table is NULL.

Prior to this patch, the tests/pbx/callerid_match test would crash during
module unload. With this patch, the test no longer crashes after running.

ASTERISK-24774 #close
Reported by: Corey Farrell

Change-Id: I2bbeecb7e0f77bac303a1b9135e4cdb4db6d4c40

main/pbx.c

@@ -10734,6 +10734,16 @@ void __ast_context_destroy(struct ast_context *list, struct ast_hashtab *context
 				exten_iter = ast_hashtab_start_traversal(tmp->root_table);
 				while ((exten_item=ast_hashtab_next(exten_iter))) {
 					int end_traversal = 1;
+
+					/*
+					 * If the extension could not be removed from the root_table due to
+					 * a loaded PBX app, it can exist here but have its peer_table be
+					 * destroyed due to a previous pass through this function.
+					 */
+					if (!exten_item->peer_table) {
+						continue;
+					}
+
 					prio_iter = ast_hashtab_start_traversal(exten_item->peer_table);
 					while ((prio_item=ast_hashtab_next(prio_iter))) {
 						char extension[AST_MAX_EXTENSION];