[gtk+] Ensure src and dest are GdkWindows in _gdk_sythesize_crossing_events

[John Ralls <jralls src gnome org>]

commit cf66cf906ec148ffbfa586b8e465d15b066412ae
Author: John Ralls <jralls ceridwen us>
Date:   Fri Oct 11 10:40:50 2013 -0700

    Ensure src and dest are GdkWindows in _gdk_sythesize_crossing_events
    
    Discovered via a crash because b's (dest's) toplevel was NULL;
    ensuring that the dest is actually a GdkWindow or setting b to NULL
    prevents that path from being taken.

 gdk/gdkwindow.c |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/gdk/gdkwindow.c b/gdk/gdkwindow.c
index 6d2bd68..b16f283 100644
--- a/gdk/gdkwindow.c
+++ b/gdk/gdkwindow.c
@@ -7570,8 +7570,9 @@ _gdk_synthesize_crossing_events (GdkDisplay                 *display,
 
   /* TODO: Don't send events to toplevel, as we get those from the windowing system */
 
-  a = src;
-  b = dest;
+  a = (src && GDK_IS_WINDOW (src)) ? src : NULL;
+  b = (dest && GDK_IS_WINDOW (dest)) ? dest : NULL;
+
   if (src == dest)
     return; /* No crossings generated between src and dest */