CVE-2017-13044/HNCP: add DHCPv4-Data bounds checks

dhcpv4_print() in print-hncp.c had the same bug as dhcpv6_print(), apply
a fix along the same lines.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s).
hncp_print(netdissect_options *ndo,
           const u_char *cp, u_int length)
{
    ND_PRINT((ndo, "hncp (%d)", length));
    hncp_print_rec(ndo, cp, length, 1);
}

hncp_print(netdissect_options *ndo,
           const u_char *cp, u_int length)
{
    ND_PRINT((ndo, "hncp (%d)", length));
    hncp_print_rec(ndo, cp, length, 1);
}
