Fix crystal ball use after being destroyed

Crystal ball can be destroyed when used, so pass the object parameter
back as null in that case.

src/artifact.c

@@ -1401,13 +1401,13 @@ doinvoke()
 
 STATIC_OVL int
 arti_invoke(obj)
-register struct obj *obj;
+struct obj *obj;
 {
     register const struct artifact *oart = get_artifact(obj);
 
     if (!oart || !oart->inv_prop) {
         if (obj->otyp == CRYSTAL_BALL)
-            use_crystal_ball(obj);
+            use_crystal_ball(&obj);
         else
             pline1(nothing_happens);
         return 1;