fix #Q404 - monster wielding cursed corpse

From a bug report, applying a bullwhip
towards a monster to try to steal its weapon would report that a cursed
cockatrice corpse was welded to the monster's hands even though corpses
wielded by the player never become welded.  Code for monsters deciding
what to wield knew that corpses don't weld; everywhere else seems to
assume that they only wield weldable weapons.  Add a routine to check
whether a wielded item is welded, similar to what's done for the hero.  I
fixed a couple of other spots besides use_whip() but didn't hunt all over.

src/muse.c

@@ -2210,19 +2210,21 @@ mcould_eat_tin(mon)
 struct monst *mon;
 {
 	struct obj *obj, *mwep;
+	boolean welded_wep;
 
 	/* monkeys who manage to steal tins can't open and eat them
 	   even if they happen to also have the appropriate tool */
 	if (is_animal(mon->data)) return FALSE;
 
 	mwep = MON_WEP(mon);
+	welded_wep = mwep && mwelded(mwep);
 	/* this is different from the player; tin opener or dagger doesn't
 	   have to be wielded, and knife can be used instead of dagger
 	   (even so, non-nymphs don't pick up tins, so only nymphs might
 	   end up being able to benefit from them) */
 	for (obj = mon->minvent; obj; obj = obj->nobj) {
 	    /* if stuck with a cursed weapon, don't check rest of inventory */
-	    if (mwep && mwep->cursed && obj != mwep) continue;
+	    if (welded_wep && obj != mwep) continue;
 
 	    if (obj->otyp == TIN_OPENER ||
 		(obj->oclass == WEAPON_CLASS &&