[qtgui] fix crash when QWindow gets deleted. Fixes JB#42211

Earlier crash fix on commit ad9eb80 caused another:
~QWindow() calls destroy() and if
QWindowSystemInterfacePrivate::handleWindowSystemEvent()
does asynchronous processing, QGuiApplicationPrivate::focus_window
will be already deleted at the time focus change gets handled.

Quite likely that focus change signaling isn't really proper with
this, but then again it's not crashing and hasn't been proper earlier
either.

src/gui/kernel/qwindow.cpp

@@ -203,6 +203,8 @@ QWindow::QWindow(QWindowPrivate &dd, QWindow *parent)
 QWindow::~QWindow()
 {
     destroy();
+    if (QGuiApplicationPrivate::focus_window == this)
+        QGuiApplicationPrivate::focus_window = 0;
     QGuiApplicationPrivate::window_list.removeAll(this);
     if (!QGuiApplicationPrivate::is_app_closing)
         QGuiApplicationPrivate::instance()->modalWindowList.removeOne(this);