SFTP: update for 3.0 branch
SFTP: update for 3.0 branch

in 1.0 / 2.0 _get_channel_packet could return false even when the
channel status was NET_SSH2_MSG_CHANNEL_DATA. in 3.0 it can't.
where 1.0 / 2.0 returned false 3.0 now throws exceptions

phpseclib/phpseclib,terrafrost/phpseclib