gssh: Avoid crash if client holds a ref to output stream on shutdown
gssh: Avoid crash if client holds a ref to output stream on shutdown

It's possible for the main connection object to be gone, but the
client app still holds a ref to the GOutputStream half, and calls say
g_output_stream_close (outstream, NULL, NULL) on it.  In that case,
just return an error immediately rather than segfaulting.

GNOME/libgssh,GNOME/libgssh,GNOME/libgssh