b2b_logic: unlink entity in the tuple before destroying

This prevents a crash generated while trying to iterate through the
tuple bridging_entities in the local route - since the entity has been
destroyed, but not removed from the tuple, accessing it may cause in
invalid memory access and crashes.

Thanks go to @BTa7BxrHYn on GitHub for reporting it
Close #3240

modules/b2b_logic/bridging.c

@@ -2147,6 +2147,7 @@ int b2bl_bridge_msg(struct sip_msg* msg, str* key, int entity_no, str *adv_ct)
 	}
 	old_entity->peer = NULL;
 
+	tuple->bridge_entities[(entity_no?0:1)] = NULL;
 	/* remove the disconected entity from the tuple */
 	if(0 == b2bl_drop_entity(old_entity, tuple))
 	{