telepathy-gabble: denial of service

From the Red Hat bugzilla:

So we have a remotely-triggered DoS: send Gabble a <presence> with a caps hash; include a form with an anonymous fixed field in the reply; boom. Since anyone can send presence to anyone else, and Gabble always looks up any caps it sees in any presences it receives. (Note that this is a presence leak, too; another bug, I think.)