Qt: Fix a possible crash on shutdown

If the game list is being refreshed due to play time update.

src/duckstation-qt/mainwindow.cpp

@@ -106,6 +106,7 @@ MainWindow::~MainWindow()
 {
   Assert(!m_display_widget);
   Assert(!m_debugger_window);
+  cancelGameListRefresh();
 
   // we compare here, since recreate destroys the window later
   if (g_main_window == this)