fix redirect-gateway behaviour when an IPv4 default route does not exist

fix redirect-gateway behaviour when an IPv4 default route does not exist

When no IPv4 default route exists, the "redirect-gateway" routine
aborts even if the sub-option "local" was specified or if we are
connecting to the remote host using IPv6.

This is not expected because in either case OpenVPN should not
bother checking the existence of the default route as it is not
required at all.

Therefore, skip the IPv4 default route check when "local" is
specified or we are connecting to an IPv6 remote host.

(This is a cherry-pick of 14670a9d654b (master), adapted to 2.3 code)

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20170509152422.12606-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg14602.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>