Fix a crash when connection-sharing during userauth.

Fix a crash when connection-sharing during userauth.

If a sharing downstream disconnected while we were still in userauth
(probably by deliberate user action, since such a downstream would
have just been sitting there waiting for upstream to be ready for it)
then we could crash by attempting to count234(ssh->channels) before
the ssh->channels tree had been set up in the first place.

A simple null-pointer check fixes it. Thanks to Antti Seppanen for the
report.