Fix turret crash

Fixes #5094
When a turret kiled a monster, it would set monster.killer to point to the fake NPC used for the turret, which was on the stack.  When the monster was cleaned up it would try to access the killer for bookkeepping, which was then an invalid pointer.
Added a member used to identify fake creatures, and used it to prevent storing the reference to the creature if it killed something.
Suggestions for a better way to do this welcome.