Avoid another pacification crash.

When reentering a level that had a confused pacified monsted on stairs:
  http://dobrazupa.org/morgue/Farquar/crash-Farquar-20141126-194317.txt

We should really just avoid removing the monster in handle_behaviour and
do that on the monster's next turn.  Similar crash fixes for pacified
monsters include 5b05a17, d65c940, and 48bc299, and those are just the
ones I committed myself.