Bug 1824152: Return success if SocketThreadService was already shutting down r=necko-reviewers,valentin

Differential Revision: https://phabricator.services.mozilla.com/D173634

netwerk/base/nsSocketTransportService2.cpp

@@ -809,14 +809,11 @@ nsSocketTransportService::Shutdown(bool aXpcomShutdown) {
 
   NS_ENSURE_STATE(NS_IsMainThread());
 
-  if (!mInitialized) {
+  if (!mInitialized || mShuttingDown) {
+    // We never inited, or shutdown has already started
     return NS_OK;
   }
 
-  if (mShuttingDown) {
-    return NS_ERROR_UNEXPECTED;
-  }
-
   {
     auto observersCopy = mShutdownObservers;
     for (auto& observer : observersCopy) {