Add some more checks to sanity_check_single_mon(). If mon->data
is discovered to be bad, panic instead of just issuing a warning
since a subsequent crash would be inevitable. Make sure hidden
ceiling hiders have a ceiling to hide at (so not on the planes of
air or water; some quest levels should probably be classified as
"no ceiling" but currently aren't). Perform a few mimic checks.
Protection from shape changers had a couple of minor bugs. A mimic
hidden at a spot the hero couldn't see would be allowed to remain
hidden (and stay that way once within view because protection from
shape changers isn't re-checked during ordinary activity). Also,
if a pet was shape-changed while eating a mimic corpse at the time
protection from shape changers started, it would fall into untimed
sleep as part of being forced back to normal shape [rescham()] if
its location could be seen.
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