Bug 824723: Don't fire error events if the worker is shutting down. r=bent

dom/workers/WorkerPrivate.cpp

@@ -937,9 +937,13 @@ public:
   bool
   WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate)
   {
-    JSObject* target = aWorkerPrivate->IsAcceptingEvents() ?
-                       aWorkerPrivate->GetJSObject() :
-                       nullptr;
+    // Don't fire this event if the JS object has been disconnected from the
+    // private object.
+    if (!aWorkerPrivate->IsAcceptingEvents()) {
+      return true;
+    }
+
+    JSObject* target = aWorkerPrivate->GetJSObject();
 
     uint64_t innerWindowId;