Reported by Vivit-R with comments by several others. The prize item
in one of the closets off the Sokoban treasure zoo is sometimes
missing, most likely picked up by an elf who won't be dissuaded by
the presence of engraved Elbereth or a scroll of scare monster.
This fix prevents monsters from targetting the mines' and sokoban's
prizes for pickup (or for eating). Once the hero picks either of the
prizes up, they stop being prizes and will be ordinary monster fodder
if dropped/stolen/stashed.
One of the comments by copperwater suggested this approach as a
possible way to fix things. I had already implemented it from scratch
before noticing that. It handles the usual monster behavior toward
items, but there could easily be some unusual cases still susceptible
to taking the prize before the hero gets to it. Those are the breaks.
Fixes#603
d736cbd751