Someone in the newsgroup mentioned that there is no warning from blessed
food detection if you attempt to eat a tin of Medusa meat. touch_petrifies()
isn't enough to catch that. A warning was given about Medusa's corpse, but
only if you lacked poison resistance. Eating a tainted Medusa corpse had a
similar problem; the check to override food poisoning so that petrification
takes effect missed it. This makes the petrification checks be consistent,
including hypothetical case of Medusa egg.