fix #H6597 - genocide exploit

fix #H6597 - genocide exploit

Self-genocide (own role or race) while polymorphed sets u.uhp to -1
so that you'll be killed during rehumanization.  I found a couple
of places which were testing (u.uhp < 1) without checking polymorph
state, and one of those was where monster movement decides whether or
not to attack.  This bug seems to have been present since start of
the second cvs repository, so has been around for quite a long time
without anybody letting on that they'd noticed.  So it probably isn't
a very effective exploit, although it would certainly make ascending
without wearing armor become much more feasible.

There are bound to be other places which examine u.uhp directly
instead of '(Upolyd ? u.mh : u.uhp)' but I only checked m*.c.