Follow-up to rev bed748189cd0 to fix a compiler warning. r=njn via IRC.

js/src/jsobj.cpp

@@ -6464,10 +6464,12 @@ js_SetReservedSlot(JSContext *cx, JSObject *obj, uint32 index, jsval v)
         return true;
 
     JSClass *clasp = obj->getClass();
-    uint32 limit = JSCLASS_RESERVED_SLOTS(clasp);
 
     JS_LOCK_OBJ(cx, obj);
+#ifdef DEBUG
+    uint32 limit = JSCLASS_RESERVED_SLOTS(clasp);
     JS_ASSERT(index < limit || ReservedSlotIndexOK(cx, obj, clasp, index, limit));
+#endif
 
     uint32 slot = JSSLOT_START(clasp) + index;
     if (slot >= JS_INITIAL_NSLOTS && !obj->dslots) {