[at-spi2-core] Don't crash if we get a object:children-changed event with a non-existent child

[Mike Gorse <mgorse src gnome org>]

commit a614d447f5a38481e700e5b2e1635692968a0fb9
Author: Owen W. Taylor <otaylor fishsoup net>
Date:   Thu Oct 1 12:43:23 2015 -0400

    Don't crash if we get a object:children-changed event with a non-existent child
    
    If we get a :children-changed event with the path for the child not matching
    any accessible in the application, event->any_data would end up with a NULL
    child, triggering a crash.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=755951

 atspi/atspi-event-listener.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/atspi/atspi-event-listener.c b/atspi/atspi-event-listener.c
index 7746ea1..be7d3e9 100644
--- a/atspi/atspi-event-listener.c
+++ b/atspi/atspi-event-listener.c
@@ -207,6 +207,8 @@ cache_process_children_changed (AtspiEvent *event)
     return;
 
   child = g_value_get_object (&event->any_data);
+  if (child)
+    return;
 
   if (!strncmp (event->type, "object:children-changed:add", 27))
   {