[at-spi2-core] Add a NULL check into atspi_event_listener_register_from_callback

[Mike Gorse <mgorse src gnome org>]

commit d08bc3d2a0d4c56cb14c782962012e2f06c7ec34
Author: Mike Gorse <mgorse novell com>
Date:   Fri Jan 28 08:28:19 2011 -0600

    Add a NULL check into atspi_event_listener_register_from_callback

 atspi/atspi-event-listener.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)
---
diff --git a/atspi/atspi-event-listener.c b/atspi/atspi-event-listener.c
index 0083c61..e931ba1 100644
--- a/atspi/atspi-event-listener.c
+++ b/atspi/atspi-event-listener.c
@@ -504,6 +504,12 @@ atspi_event_listener_register_from_callback (AtspiEventListenerCB callback,
       return FALSE;
     }
 
+  if (!event_type)
+  {
+    g_warning (_("called atspi_event_listener_register_from_callback with a NULL event_type"));
+    return FALSE;
+  }
+
   e = g_new (EventListenerEntry, 1);
   if (!e) return FALSE;
   e->callback = callback;