Don't let Riders swap places with something (fog or ooze, perhaps)
located at a closed door spot because if it gets killed there, there
won't be any corpse and it will stop auto-reviving.
Just avoid moving to spots where mondied() won't place a corpse
instead of worrying about whether a bargethrough creature (if there
ever are any besides the Riders) might be able to survive at the
destination (so ignore pass-walls, door-opening, swimming, &c).
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