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/detect.c

@@ -891,11 +891,12 @@ static const struct {
 };
 
 void
-use_crystal_ball(obj)
-struct obj *obj;
+use_crystal_ball(optr)
+struct obj **optr;
 {
     char ch;
     int oops;
+    struct obj *obj = *optr;
 
     if (Blind) {
         pline("Too bad you can't see %s.", the(xname(obj)));
@@ -930,7 +931,7 @@ struct obj *obj;
         case 5:
             pline("%s!", Tobjnam(obj, "explode"));
             useup(obj);
-            obj = 0; /* it's gone */
+            *optr = obj = 0; /* it's gone */
             /* physical damage cause by the shards and force */
             losehp(Maybe_Half_Phys(rnd(30)), "exploding crystal ball",
                    KILLED_BY_AN);