extra NULL check

It is undefined behavior what happens when you try to call pthread_join on a NULL thread (which will happen if you close the game before opening the save browser). Seems to be harmless, but make sure we don't do that just in case --jacob1

src/client/DownloadManager.cpp

@@ -38,7 +38,8 @@ void DownloadManager::Shutdown()
 	managerShutdown = true;
 	pthread_mutex_unlock(&downloadAddLock);
 	pthread_mutex_unlock(&downloadLock);
-	pthread_join(downloadThread, NULL);
+	if (downloadThread)
+		pthread_join(downloadThread, NULL);
 }
 
 //helper function for download