Fix NULL-pointer crash in route_list_add_vpn_gateway().

Add ASSERT() check to route_list_add_vpn_gateway() to ensure that *rl
is valid (and if not, crash with a somewhat more meaningful message than
"segmentation violation").  For the actual bugfix, change init code to
always allocate a "struct route_list" for IPv4, even if no --route options
have been seen in the config.

Fix trac#281 and trac#258.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Josh Cepek <josh.cepek@usa.net>
Message-Id: <1367608404-2257-1-git-send-email-gert@greenie.muc.de>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7581
(cherry picked from commit eb95f367348f4c2aae301cfa7c3adc8e0f2e711e)