Bug 1539208 - P1 - Return an error when the Reponse object is null'd; r=mayhemer

Differential Revision: https://phabricator.services.mozilla.com/D25228

--HG--
extra : moz-landing-system : lando

dom/fetch/FetchDriver.cpp

@@ -1048,7 +1048,7 @@ FetchDriver::OnStartRequest(nsIRequest* aRequest) {
     // Fail to generate a paddingInfo for opaque response.
     MOZ_DIAGNOSTIC_ASSERT(mResponse->Type() == ResponseType::Opaque);
     FailWithNetworkError(NS_ERROR_UNEXPECTED);
-    return rv;
+    return NS_ERROR_UNEXPECTED;
   }
 
   // From "Main Fetch" step 19: SRI-part1.