res_pjsip_mwi: Remove inappropriate topic unreference.

ast_mwi_topic() returns a borrowed reference which should not be
unreferenced, doing so leads to a FRACK.  This was hidden by the fact
that stasis_cache.c leaked the result of cache_remove in
caching_topic_exec.

Change-Id: I51101bf7d07b8dc8ce8fc46b6cb31fbbd213fbc7

main/stasis_cache.c

@@ -868,7 +868,7 @@ static void caching_topic_exec(void *data, struct stasis_subscription *sub,
 			ao2_wrlock(caching_topic->cache->entries);
 			sub = cache_find(caching_topic->cache->entries, stasis_subscription_change_type(), change->uniqueid);
 			if (sub) {
-				cache_remove(caching_topic->cache->entries, sub, stasis_message_eid(message));
+				ao2_cleanup(cache_remove(caching_topic->cache->entries, sub, stasis_message_eid(message)));
 				ao2_cleanup(sub);
 			}
 			ao2_unlock(caching_topic->cache->entries);

res/res_pjsip_mwi.c

@@ -266,7 +266,6 @@ static struct mwi_stasis_subscription *mwi_stasis_subscription_alloc(const char
 		mailbox, mwi_sub->id, stasis_topic_name(topic), topic, (int)ao2_ref(topic, 0));
 	ao2_ref(mwi_sub, +1);
 	mwi_stasis_sub->stasis_sub = stasis_subscribe_pool(topic, mwi_stasis_cb, mwi_sub);
-	ao2_ref(topic, -1);
 
 	if (!mwi_stasis_sub->stasis_sub) {
 		/* Failed to subscribe. */