xcb: another QXcbScreen null pointer check

In QXcbWindow::setParent(), the window may not have a screen,
and in that case we cannot get the root window in this way.

Task-number: QTBUG-44719
Change-Id: I719e5e2f8cad13b1460b4d9df6ffd6c4a48e0d37
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -1371,6 +1371,8 @@ void QXcbWindow::setParent(const QPlatformWindow *parent)
         xcb_parent_id = qXcbParent->xcb_window();
         m_embedded = qXcbParent->window()->type() == Qt::ForeignWindow;
     } else {
+        if (!xcbScreen())
+            return;
         xcb_parent_id = xcbScreen()->root();
         m_embedded = false;
     }