Crash when falling monster dies from damage

[RMTEW FULL NAME]

Report from Bay12 forum. Basically there was an island surrounded by a chasm and spawning monsters onto the island would cause monsters to fall off and down a level. They'd die when they hit the ground, but the falling logic would not check if they died, and would crash when it tried to proceed.

[RMTEW FULL NAME]

If a monster moves over a chasm, then dies falling, there was no check whether this happened and things proceeded like the monster survived. Crashing happened instead. Closes #49

→ <<cset 92a5319d1696>>