ICMP_ID is u_int16_t not u_int8_t. Handle this properly.

Signed-off-by: Krzysztof Piotr Oledzki <ole@ans.pl>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

-- 
The dawn of the fourth age of Linux firewalling is coming; a time of
great struggle and heroic deeds -- J.Kadlecsik got inspired by J.Morris