Don't error on `SSH_AGAIN` in `discard_output`

[itsfarseen]

As per the docs of ssh_channel_read_timeout:

The number of bytes read, 0 on end of file or SSH_ERROR on error. In nonblocking mode it can return 0 if no data is available or SSH_AGAIN.

SSH_AGAIN is -2. So it was hitting the nbytes < 0 branch and exiting with an error. Changed it to have the same behaviour as nbytes == 0

Reference: https://api.libssh.org/master/group__libssh__channel.html#ga2f4e02cb3b3adbc30a534623164068fd