Fix the bug introduced in r342908, that causes problems with dynamic
handling for protocols without ports numbers.

Since port numbers were uninitialized for protocols like ICMP/ICMPv6,
ipfw_chk() used some non-zero values to create dynamic states, and due
this it failed to match replies with created states.

Reported by: Oliver Hartmann, Boris Lytochkin
Obtained from: Yandex LLC
X-MFC after: r342908