a11y: Fix warnings when removing widgets that don't have accessibles

The remove handler would not check that removed widgets do indeed have
an accessible associated with them and would happily run all the removal
code with a NULL object. Not good.

gtk/a11y/gtkcontaineraccessible.c

@@ -142,6 +142,8 @@ gtk_container_accessible_real_remove_gtk (GtkContainer *container,
 
   atk_parent = ATK_OBJECT (data);
   atk_child = gtk_widget_get_accessible (widget);
+  if (atk_child == NULL)
+    return 1;
   accessible = GTK_CONTAINER_ACCESSIBLE (atk_parent);
 
   g_object_notify (G_OBJECT (atk_child), "accessible_parent");