Disambiguate b_trapped null bodypart value

b_trapped was treating 0 as a null value for its bodypart parameter, but
0 is actually the value of ARM, so b_trapped(..., ARM) would be treated
as intending no A_CON abuse.  Add NO_PART = -1 to the bodypart_types
enum, and use that instead of 0 as the "no body part" value in
b_trapped, so that ARM can be passed to it without any ambiguity.

aosdict identified this issue in xNetHack and handled it differently; he
added NO_PART with a value of 0, incremented the existing bodypart_types
values, and padded the body part arrays so the actual body parts would
start at index 1.  I think using NO_PART = -1 is simpler, but that's an
alternative approach that can be used instead -- it is advantageous in
that it automatically fixes any other places where 0 is assumed to be a
non-body-part value that I may have overlooked.

src/polyself.c

@@ -1988,6 +1988,11 @@ mbodypart(struct monst *mon, int part)
     };
     struct permonst *mptr = mon->data;
 
+    if (part <= NO_PART) {
+        impossible("mbodypart: bad part %d", part);
+        return "mystery part";
+    }
+
     /* some special cases */
     if (mptr->mlet == S_DOG || mptr->mlet == S_FELINE
         || mptr->mlet == S_RODENT || mptr == &mons[PM_OWLBEAR]) {