Don't try sending on sharing channels.

Don't try sending on sharing channels.

The final main loop in do_ssh2_authconn will sometimes loop over all
currently open channels calling ssh2_try_send_and_unthrottle. If the
channel is a sharing one, however, that will reference fields of the
channel structure like 'remwindow', which were never initialised in
the first place (thanks, valgrind). Fix by excluding CHAN_SHARING
channels from that loop.

(cherry picked from commit 7366fde1d4831dcc701bc31e9de1113636fba1c5)