[gh-806] Return from unl_og_membership_insert if not in OG user context

Without this fix:

REFERRER	http://arts.unl.edu/node/add/course
MESSAGE	PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'uid' cannot be null: INSERT INTO {users_roles} (uid, rid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1); Array ( [:db_insert_placeholder_0] => [:db_insert_placeholder_1] => 54 ) in unl_og_membership_insert() (line 21 of /var/www/htdocs/sites/all/modules/unl/unl.module).