Fix exception on muc-room-affiliations
If an affiliation for an entry without the '@' character is set then the "User Permissions" page will throw an StringIndexOutOfBoundsException. This prevents admins to setup an affiliation for a component (e.g. a chat bot with JID "bot.domain.lt") or delete invalid affiliations.
This was reported on http://www.igniterealtime.org/community/thread/36610
Comitted in revision 11191.