fix #H1826 - split long worm with 0 hit points

fix #H1826 - split long worm with 0 hit points

     From a bug report, a long worm with 0 HP
was observed via stethoscope after cutting one or more worms in half many
times, followed by an unspecified crash.  Cutting a worm doesn't reduce
its level below 3, but if a worm is drained to level 0 by some other means
and then gets cut in half (and still has at least 2 HP left), cutworm()
would give the new level 0 worm 0d8 (hence 0) for current and max HP.
That could confuse end-of-move monster cleanup, which thinks 0 HP is a
dead monster who has been removed from the map but not yet purged from the
fmon list.  Purging it would then leave a stale monster pointer on the map.

     cutworm() should have special cased level 0 to use 1d4 for HP, but
instead I've changed it to not produce a cloned worm if the source one is
lower than level 3.