If we're called on to uncleanly close an SSH channel for which we've

If we're called on to uncleanly close an SSH channel for which we've
already sent SSH2_MSG_CHANNEL_CLOSE, we should not skip the _whole_ of
sshfwd_unclean_close(), only the part about sending
SSH2_MSG_CHANNEL_CLOSE. It's still important to retag the SSH channel
as CHAN_ZOMBIE and clean up its previous data provider.

[originally from svn r9389]