fix #H8481 - placing monster at <0,0>

fix #H8481 - placing monster at <0,0>

mon_arrive() -> m_into_limbo() -> migrate_to_level() -> wormgone()
followed by place_monster() "for relmon".  relmon() was changed (last
November, cc5bb44a9a757fe00800c90cb3a648b009af155d) to not require
the monster be on the map, so just get rid of the place_monster() that
was trying to put the "gone" long worm at <0,0>.

Also, another m_into_limbo() bit:  make mdrop_special_objs() check the
location and send any dropped items to random locations if the monster
dropping things isn't on the map, instead of placing them at <0,0>.