ssh/channel-type.h (GSSH_VALIDATE_CHANNEL_DATA): New macro

* ssh/channel-type.h (GSSH_VALIDATE_CHANNEL_DATA): New macro.
* ssh/channel-func.c (guile_ssh_channel_is_open_p): Return `#f' if the
  channel has been closed and freed.
  (guile_ssh_channel_is_eof_p, guile_ssh_channel_open_session): Throw an
  exeption if the channel has been closed and freed.