close_sent_cb: check if close operation hasn't be completed yet
close_sent_cb: check if close operation hasn't be completed yet

If we receive the other side close before we finished to send yours, the
close operation would be already completed.

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