more object deletion bookkeeping

Monster breaking out of armor triggered the object-still-worn warning
that was added to object deletion a couple of days ago.  Update m_useupall()
to clear owornmask.

src/mthrowu.c

@@ -456,6 +456,7 @@ struct obj *obj;
 	if (obj->owornmask) {
 	    mon->misc_worn_check &= ~obj->owornmask;
 	    update_mon_intrinsics(mon, obj, FALSE, FALSE);
+	    obj->owornmask = 0L;
 	}
 	obfree(obj, (struct obj*) 0);
 }