McdDispatcher: if a client disappears, end its "readying" immediately
McdDispatcher: if a client disappears, end its "readying" immediately

Otherwise, we'll get a use-after-free when the Get("HandledChannels") call
eventually returns.

(I suspect having McdClient be a subclass of TpProxy would be a better
long-term fix for this.)

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control