From a bug report, a monster which
died by moving into a trap which was next to the hero standing on Elbereth
resulted in "The <mon> is killed! The <mon> turns to flee!". An earlier
change made monflee() return if it's given a dead monster, so the fleeing
message is no longer given. This fixes the place where monflee() was
inappropriately being called for a dead monster in the reported situation.
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