Menu deleted when group node edited by user that is not a group member

When a user who has access to edit the group node and permission to administer og menus, but is not a member of the group, edits the group node, the og_menu checkbox on the node form shows as unchecked despite that a menu may exist. Then when the user saves the node, the entire menu is silently deleted.

The patch changes the checkbox default value logic for existing nodes to prevent this.