mfmediaengine: Don't send event notify when engine is shutdown.

I've also tried saving a cancel cookie from `BeginCreateObject*` and using it to cancel the
operation in engine shutdown. However there are 2 problems:

  1. Callback might still get called after `CancelObjectCreation`. Microsoft's
     [own documentation](https://learn.microsoft.com/en-us/windows/win32/api/mfidl/nf-mfidl-imfsourceresolver-cancelobjectcreation)
     says as much:
       > Because these methods are asynchronous, however, they might be completed before the
       > operation can be canceled. Therefore, your callback might still be invoked after you
       > call this method.
  2. Our implementations of `BeginCreateObject*` are not cancellable. They call either
     `MFPutWorkItem` or `IMFByteStream_BeginRead`, neight of which provides cancellation.
     (and our `CancelObjectCreation` doesn't actually work. AsyncResult objects aren't added to
     `handler->results` list until they are completed, which means any attempts to cancel an
     yet-to-be-completed object creation will always fail with `MF_E_UNEXPECTED`)

dlls/mfmediaengine/main.c

@@ -1356,6 +1356,12 @@ static HRESULT WINAPI media_engine_load_handler_Invoke(IMFAsyncCallback *iface,
 
     EnterCriticalSection(&engine->cs);
 
+    if (engine->flags & FLAGS_ENGINE_SHUT_DOWN)
+    {
+        LeaveCriticalSection(&engine->cs);
+        return S_OK;
+    }
+
     engine->network_state = MF_MEDIA_ENGINE_NETWORK_LOADING;
     IMFMediaEngineNotify_EventNotify(engine->callback, MF_MEDIA_ENGINE_EVENT_LOADSTART, 0, 0);