[gtk+] gtknotebook: Fix critical warnings on scroll events

[Javier Jardón <jjardon src gnome org>]

commit 1af1362a369bb79deaeec8d5bf580c97df6c0c87
Author: Christian Persch <chpe gnome org>
Date:   Tue Mar 23 14:45:07 2010 +0100

    gtknotebook: Fix critical warnings on scroll events
    
    Fixes https://bugzilla.gnome.org/show_bug.cgi?id=604391

 gtk/gtknotebook.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/gtk/gtknotebook.c b/gtk/gtknotebook.c
index d2a9876..65b7ee0 100644
--- a/gtk/gtknotebook.c
+++ b/gtk/gtknotebook.c
@@ -2550,7 +2550,8 @@ gtk_notebook_scroll (GtkWidget      *widget,
   for (i = 0; i < 2; i++)
     {
       if (event_widget == priv->action_widget[i] ||
-          gtk_widget_is_ancestor (event_widget, priv->action_widget[i]))
+          (priv->action_widget[i] &&
+           gtk_widget_is_ancestor (event_widget, priv->action_widget[i])))
         return FALSE;
     }