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

@@ -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));