In some cases (mostly for one-port devices) IF_WAN was used
although not set, resulting in not obviously iptables error
messages like
- Bad argument `conntrack'
- Bad argument `REJECT'
Thus, check whether IF_WAN is set to something before using it.
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>