Bug 717173 - 'Crash @ JS_ReportPendingException'. r=khuey.

dom/workers/WorkerPrivate.cpp

@@ -1528,7 +1528,7 @@ WorkerRunnable::PreDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate)
   }
 #endif
 
-  if (mBusyBehavior == ModifyBusyCount) {
+  if (mBusyBehavior == ModifyBusyCount && aCx) {
     return aWorkerPrivate->ModifyBusyCount(aCx, true);
   }
 
@@ -1616,7 +1616,7 @@ WorkerRunnable::PostDispatch(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
   }
 #endif
 
-  if (!aDispatchResult) {
+  if (!aDispatchResult && aCx) {
     if (mBusyBehavior == ModifyBusyCount) {
       aWorkerPrivate->ModifyBusyCount(aCx, false);
     }