Fix FsParticipant leak
Fix FsParticipant leak

When getting a participant that already exists, the FsParticipant
to which it corresponds is reffed, but not unreffed in the put
function.

freedesktop-unofficial-mirror/telepathy__telepathy-farstream,freedesktop-unofficial-mirror/telepathy__telepathy-farstream,freedesktop-unofficial-mirror/telepathy__telepathy-farstream