Reported by a hardfought user, a ceiling_hider monster hid on the
Astral level, triggering a sanity check warning that gets repeated
each move until the hider comes out of hiding. Normally sanity_check
can only be set in wizard mode, but hardfought must force it on for
to-be-3.7.
I was able to reproduce it before this fix and unable to do so
afterward, but there is a random factor involved hence no guarantees.
I think that lack of ceiling for Astral is recent, but this could
have happened on other levels which lack a ceiling. I didn't try to
figure out whether it might happen with 3.6.x, just put the fixes
entry in the "exposed by git" (found in code not yet released, that
is) section.
c868feb383