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/wield.c

@@ -855,4 +855,14 @@ register struct obj *obj;
 	obj->owornmask = savewornmask;
 }
 
+/* test whether monster's wielded weapon is stuck to hand/paw/whatever */
+boolean
+mwelded(obj)
+struct obj *obj;
+{
+    /* caller is responsible for making sure this is a monster's item */
+    if (obj && (obj->owornmask & W_WEP) && will_weld(obj)) return TRUE;
+    return FALSE;
+}
+
 /*wield.c*/