Fix info.af == AF_UNSPEC case for server with --mtu-disc

Fix info.af == AF_UNSPEC case for server with --mtu-disc

Commit 2bed089d31a12c2 introduced "AF_UNSPEC" sockets when we do not know
the actual address family yet - for the "bind local" case, getaddrinfo()
will tell us what to do, but that information never made it into
sock->info.af - so, make it.  Otherwise, trying to call --mtu-disc
on an OpenVPN server will cause a M_FATAL error in set_mtu_discovery())

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Christian Pellegrin <chripell@google.com>
Message-ID: <20151121200637.GD24952@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10547