[gtk+] x11: Guard against NULL window

[Timm Bäder <baedert src gnome org>]

commit f393552fec79b0aebd77222a99b69c887214c964
Author: Timm Bäder <mail baedert org>
Date:   Tue Dec 19 07:38:35 2017 +0100

    x11: Guard against NULL window
    
    Otherwise, the NULL parent window causes a crash in the
    GDK_WINDOW_IS_DESTROYED call.

 gdk/x11/gdkwindow-x11.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gdk/x11/gdkwindow-x11.c b/gdk/x11/gdkwindow-x11.c
index f2e23df..c6d6e30 100644
--- a/gdk/x11/gdkwindow-x11.c
+++ b/gdk/x11/gdkwindow-x11.c
@@ -2453,7 +2453,7 @@ gdk_x11_window_set_transient_for (GdkWindow *window,
     return;
 
   /* XSetTransientForHint() doesn't allow unsetting, so do it manually */
-  if (!GDK_WINDOW_DESTROYED (parent))
+  if (!parent || !GDK_WINDOW_DESTROYED (parent))
     XSetTransientForHint (GDK_WINDOW_XDISPLAY (window), 
                          GDK_WINDOW_XID (window),
                          GDK_WINDOW_XID (parent));