[mutter] x11: Add window test before accessing reparents_pending field

[Jonas Ådahl <jadahl src gnome org>]

commit fdda8adfcf4fddd22051a1070ff037532cfbcf94
Author: Rémi Bernon <rbernon codeweavers com>
Date:   Mon Sep 2 18:22:40 2019 +0200

    x11: Add window test before accessing reparents_pending field
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/768

 src/x11/events.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/x11/events.c b/src/x11/events.c
index e6f8ac884..88b8fb639 100644
--- a/src/x11/events.c
+++ b/src/x11/events.c
@@ -1437,7 +1437,7 @@ handle_other_xevent (MetaX11Display *x11_display,
       break;
     case ReparentNotify:
       {
-        if (window->reparents_pending > 0)
+        if (window && window->reparents_pending > 0)
           window->reparents_pending -= 1;
         if (event->xreparent.event == x11_display->xroot)
           meta_stack_tracker_reparent_event (display->stack_tracker,