Fix segfault when ghost dies from a death pit

When a ghost dies from a death pit, the corresponding mobj is removed even though ghosts keep going after touching one. This triggers a segfault when the game tries to update the now-deleted mobj that the ghost corresponds to. This PR simply fixes this by just ignoring MT_GHOST when a ghost touches a death pit, allowing it to continue as intended.

The easiest way to reproduce this is by simply jumping into a death pit in record attack mode, and then starting a new record attack run with the last played ghost set to shown. Once the ghost touches the death pit, the game segfaults.

Fixes #999 (closed)