Fix proto tcp6 for server & non-P2MP modes

This fix adds support for using tcp6 as a proto in server or non-P2MP
modes, resolving a failed ASSERT in such cases.

Fixes the crash in trac#288.

Signed-off-by: Josh Cepek <josh.cepek@usa.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <klul5d$8ta$1@ger.gmane.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7578
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit d0ccb982e1714c8dfefd6eacf0c6f899eb71b582)