If the SSH server sends us CHANNEL_CLOSE for a channel on which we're

If the SSH server sends us CHANNEL_CLOSE for a channel on which we're
sitting on a pile of buffered data waiting for WINDOW_ADJUSTs, we
should throw away that buffered data, because the CHANNEL_CLOSE tells
us that we won't be receiving those WINDOW_ADJUSTs, and if we hang on
to the data and keep trying then it'll prevent ssh_channel_try_eof
from sending the CHANNEL_EOF which is a prerequisite of sending our
own CHANNEL_CLOSE.

[originally from svn r9953]