fix fatal error when renaming new group with no users

When a new group is created, a respective `group:groupName:members` is not created until a new member is added.

When `db.rename` is executed on a new group, `next` is called with `err` set, so the last function in the `async#series` is not called. This will result in the database containing `group:newName` and `oldName` in the `groups` set.

`Groups.get` with then throw `'group-not-found'` and callback with an erroneous `results.base == [, undefined]`, because `db.getObject('group:' + groupName, function(err, groupObj))` will callback with an undefined `groupObj`, affecting functions such as `Groups.list`.