Qt: Fix rare crash during update download

Thanks to Silent for debugging.

src/duckstation-qt/autoupdaterdialog.cpp

@@ -440,6 +440,10 @@ void AutoUpdaterDialog::downloadUpdateClicked()
     },
     &progress);
 
+  // Since we're going to block, don't allow the timer to poll, otherwise the progress callback can cause the timer to
+  // run, and recursively poll again.
+  m_http_poll_timer->stop();
+
   // Block until completion.
   while (m_http->HasAnyRequests())
   {