Shouldn't add user 0 to the mandatory group

In some situations when the mandatory group module is used in combination with a user access or authentication module, such as openid, the anonymous user is added to the mandatory group. This appears to be caused because an account with uid 0 is being passed around prior to the account being properly setup. If the og_mandatory_group_user hook is executed before the module that is setting up the user, then user 0 may be added to the mandatory group.

The attached patch prevents this from happening and writes a watchdog message if it tries to.