make Creature_tracker::update_pos handle dead critters, too

creature_tracker: check for dead critters if the critter to be moved is dead. Adds the private function `dead_mon_at` to get the id of dead critters.

All the usual checks & debug messages are still there and functional. If the source location does not even contain a dead critter, the debug message will still come up. It should be fairly noninvasive for the rest of the code.

fixes #5446
fixes #5160