Fix ball and chain sanity

Scatter did not consider the ball or chain, and moved them around, causing
ball and chain sanity error.

One way to trigger was being punished, with chain on a land mine and having
a monster trigger the mine. Now the chain will shatter, unpunishing the hero.

src/explode.c

@@ -621,6 +621,13 @@ struct obj *obj; /* only scatter this obj        */
                    obj->ox, obj->oy, sx, sy);
 
     while ((otmp = (individual_object ? obj : level.objects[sx][sy])) != 0) {
+        if (otmp == uball || otmp == uchain) {
+            boolean waschain = (otmp == uchain);
+            pline_The("chain shatters!");
+            unpunish();
+            if (waschain)
+                continue;
+        }
         if (otmp->quan > 1L) {
             qtmp = otmp->quan - 1L;
             if (qtmp > LARGEST_INT)