_mcd_channel_request_proceed: don't allow for absent accounts
_mcd_channel_request_proceed: don't allow for absent accounts

In earlier versions, this could possibly have happened if the code was
tangled enough. However, McdRequest holds a reference to the account,
so there's no way a McdRequest can not have an account.

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