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.

include/hack.h

@@ -132,6 +132,7 @@ enum bhit_call_types {
 
 /* Macros for messages referring to hands, eyes, feet, etc... */
 enum bodypart_types {
+    NO_PART   = -1,
     ARM       =  0,
     EYE       =  1,
     FACE      =  2,

src/detect.c

@@ -1640,7 +1640,7 @@ openone(coordxy zx, coordxy zy, genericptr_t num)
             cvt_sdoor_to_door(&levl[zx][zy]); /* .typ = DOOR */
         if (levl[zx][zy].doormask & D_TRAPPED) {
             if (distu(zx, zy) < 3)
-                b_trapped("door", 0);
+                b_trapped("door", NO_PART);
             else
                 Norep("You %s an explosion!",
                       cansee(zx, zy) ? "see" : (!Deaf ? "hear"

src/dig.c

@@ -454,7 +454,7 @@ dig(void)
         }
         if (IS_DOOR(lev->typ) && (lev->doormask & D_TRAPPED)) {
             lev->doormask = D_NODOOR;
-            b_trapped("door", 0);
+            b_trapped("door", NO_PART);
             newsym(dpx, dpy);
         }
  cleanup:

src/eat.c

@@ -1465,7 +1465,7 @@ consume_tin(const char *mesg)
 
     r = tin_variety(tin, FALSE);
     if (tin->otrapped || (tin->cursed && r != HOMEMADE_TIN && !rn2(8))) {
-        b_trapped("tin", 0);
+        b_trapped("tin", NO_PART);
         tin = costly_tin(COST_DSTROY);
         goto use_up_tin;
     }

src/hack.c

@@ -693,7 +693,7 @@ still_chewing(coordxy x, coordxy y)
     } else if (lev->typ == SDOOR) {
         if (lev->doormask & D_TRAPPED) {
             lev->doormask = D_NODOOR;
-            b_trapped("secret door", 0);
+            b_trapped("secret door", NO_PART);
         } else {
             digtxt = "chew through the secret door.";
             lev->doormask = D_BROKEN;
@@ -707,7 +707,7 @@ still_chewing(coordxy x, coordxy y)
         }
         if (lev->doormask & D_TRAPPED) {
             lev->doormask = D_NODOOR;
-            b_trapped("door", 0);
+            b_trapped("door", NO_PART);
         } else {
             digtxt = "chew through the door.";
             lev->doormask = D_BROKEN;

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]) {

src/trap.c

@@ -6265,7 +6265,7 @@ b_trapped(const char* item, int bodypart)
     wake_nearby();
     losehp(Maybe_Half_Phys(dmg), "explosion", KILLED_BY_AN);
     exercise(A_STR, FALSE);
-    if (bodypart)
+    if (bodypart != NO_PART)
         exercise(A_CON, FALSE);
     make_stunned((HStun & TIMEOUT) + (long) dmg, TRUE);
 }