Ensure that vpnc-script-win.js works even if INTERNAL_IP4_{NETADDR,NETMASK} are unset

Ensure that vpnc-script-win.js works even if INTERNAL_IP4_{NETADDR,NETMASK} are unset

Not all protocols supported by OpenConnect consistently set the variables
INTERNAL_IP4_{NETADDR,NETMASK} in all cases.

This should be fixed within OpenConnect itself as part of
https://gitlab.com/openconnect/openconnect/-/merge_requests/215, but in the
meantime (and for older versions of OpenConnect/vpnc) we need to ensure that
vpnc-script-win.js works correctly even when these are unset.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>