PTP: Use ND_LCHECK_U for checking invalid length

[tcpdump] / tests / mtrace.out

    1  06:10:40.113730 IP 10.0.0.5 > 172.16.20.1: mtrace 7: 172.16.40.1 to 172.16.20.1 reply-to 172.16.40.1
    2  06:10:40.137809 IP 10.0.0.6 > 10.0.0.5: mtrace 7: 172.16.40.1 to 172.16.20.1 reply-to 172.16.40.1