[mutter] compositor: Avoid a crash if the top window actor is finalized

[Rui Matos <rtcm src gnome org>]

commit 3caefd8fdac71d6d82961f1ceb471452692430b4
Author: Rui Matos <tiagomatos gmail com>
Date:   Tue Oct 17 17:17:55 2017 +0200

    compositor: Avoid a crash if the top window actor is finalized
    
    Since we're not holding a reference, the top window actor might be
    finalized when we paint resulting in a use after free crash.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=788493

 src/compositor/compositor.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/src/compositor/compositor.c b/src/compositor/compositor.c
index a75ac12..1d5b9ab 100644
--- a/src/compositor/compositor.c
+++ b/src/compositor/compositor.c
@@ -671,6 +671,9 @@ meta_compositor_remove_window (MetaCompositor *compositor,
   if (compositor->unredirected_window == window)
     set_unredirected_window (compositor, NULL);
 
+  if (compositor->top_window_actor == window_actor)
+    compositor->top_window_actor = NULL;
+
   meta_window_actor_destroy (window_actor);
 }