Bug 1660555 - Null mSignalProxy after calling Shutdown in WorkerFetchResolver. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D109605

dom/fetch/Fetch.cpp

@@ -358,6 +358,7 @@ class WorkerFetchResolver final : public FetchDriverObserver {
 
     if (mSignalProxy) {
       mSignalProxy->Shutdown();
+      mSignalProxy = nullptr;
     }
 
     mWorkerRef = nullptr;