Qt: Prevent crash when closed by the window manager

Source/Core/DolphinQt/MainWindow.cpp

@@ -92,6 +92,11 @@ DMainWindow::~DMainWindow()
 {
 }
 
+void DMainWindow::closeEvent(QCloseEvent* ce)
+{
+	Stop();
+}
+
 // Emulation
 
 void DMainWindow::StartGame(const QString filename)

Source/Core/DolphinQt/MainWindow.h

@@ -63,6 +63,7 @@ private slots:
 	void UpdateIcons();
 
 private:
+	void closeEvent(QCloseEvent* ce);
 	std::unique_ptr<Ui::DMainWindow> m_ui;
 	DGameTracker* m_game_tracker;