shell: Fix crash when using keynav in overview

get_item_views() was expecting all the children of the overview
to be of type "CcShellCategoryView".

It's not the case since 620e70113c
added GtkSeparators as children.

We now just skip them non-views in get_item_views().

https://bugzilla.gnome.org/show_bug.cgi?id=677314

shell/gnome-control-center.c

@@ -306,6 +306,8 @@ get_item_views (GnomeControlCenter *shell)
   res = NULL;
   for (l = list; l; l = l->next)
     {
+      if (!CC_IS_SHELL_CATEGORY_VIEW (l->data))
+        continue;
       res = g_list_append (res, cc_shell_category_view_get_item_view (CC_SHELL_CATEGORY_VIEW (l->data)));
     }