[gtk/gtk-3-24] x11: Add a precondition

[Matthias Clasen <matthiasc src gnome org>]

commit 9baae419f6eb6089dc4d03edbb26b814d937543d
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Sep 9 14:50:08 2022 -0400

    x11: Add a precondition
    
    Trying to be helpful.

 gdk/x11/gdkwindow-x11.c | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/gdk/x11/gdkwindow-x11.c b/gdk/x11/gdkwindow-x11.c
index eb822e0989..194bc82e29 100644
--- a/gdk/x11/gdkwindow-x11.c
+++ b/gdk/x11/gdkwindow-x11.c
@@ -5641,6 +5641,8 @@ gdk_x11_get_server_time (GdkWindow *window)
 XID
 gdk_x11_window_get_xid (GdkWindow *window)
 {
+  g_return_val_if_fail (GDK_IS_X11_WINDOW (window), None);
+
   /* Try to ensure the window has a native window */
   if (!_gdk_window_has_impl (window))
     {