SaslAuth: Don't process stanzas received after disconnection
SaslAuth: Don't process stanzas received after disconnection

Fixes: <https://bugs.freedesktop.org/show_bug.cgi?id=35430>
Reviewed-by: Will Thompson <5822fdda638242a44ccfa68f146ccf6f257c5cbd@collabora.co.uk>

noonien-d/wocky,freedesktop-unofficial-mirror/wocky,noonien-d/wocky,freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky,noonien-d/wocky