Bug 1424647: Prevent race on AllPromiseHolder::mPromise. r=jwwang

If any of the promises gets resolved/rejected prior MozPromise::All completing, it would return nullptr

MozReview-Commit-ID: Lqhv2t2upvF

--HG--
extra : rebase_source : 0c85172958c00a2ac3aa0bd33e4e50dd1893c3ba

xpcom/threads/MozPromise.h

@@ -341,13 +341,14 @@ public:
     }
 
     RefPtr<AllPromiseHolder> holder = new AllPromiseHolder(aPromises.Length());
+    RefPtr<AllPromiseType> promise = holder->Promise();
     for (size_t i = 0; i < aPromises.Length(); ++i) {
       aPromises[i]->Then(aProcessingTarget, __func__,
         [holder, i] (ResolveValueType aResolveValue) -> void { holder->Resolve(i, Move(aResolveValue)); },
         [holder] (RejectValueType aRejectValue) -> void { holder->Reject(Move(aRejectValue)); }
       );
     }
-    return holder->Promise();
+    return promise;
   }
 
   class Request : public MozPromiseRefcountable