Fix clang warning in options.c

Fix clang warning in options.c

fixed warning: expression which evaluates to zero treated as a
null pointer constant of type 'struct addrinfo *'

Seems to be innocent, but clang is correct that this is strange.
init_tun() expects two pointers, but options_string() tried to
feed it two uint32_t values.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: David Sommerseth <dazo@users.sourceforge.net>
Message-Id: <1408568426-19601-2-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9004
Signed-off-by: Gert Doering <gert@greenie.muc.de>