Issue reported by mkuoppal: drum of earthquake triggers a sanity
check warning which the fuzzer escalated to panic.
Analysis by entrez. If a pit gets created next to a pool or moat or
lava, liquid might flow there and replace the pit. But the drum of
earthquake code assumed that the pit was still there. If there was a
monster there and it wasn't levitating or flying and it wasn't killed
by the liquid, it got marked as trapped even though the pit was gone.
'sanity_check' noticed.
With difficulty I was able to reproduce the impossible warning before
the fix, but the are a bunch of random factors at play. After the
fix I can't reproduce it again, but that's not a guarantee that it's
actually fixed. The analysis seems correct though and the fix is
based on dealing with that.
Closes#1170
31717fe227