Fix DecrementTTL option.

The option was not actually working, as it could be seen on traceroute or mtr.

The problem is that it was checking if the TTL was < 1 (so equal to 0) before decrementing it.

This meant that a packet with a TTL of 1 was being sent with a TTL of 0 on the VPN, instead of being discarded with the ICMP error message.

Signed-off-by: Vittorio Gambaletta <openwrt@vittgam.net>