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).
is_in_line(netdissect_options *ndo, int indent)
{
    return indent - 1 >= ndo->ndo_vflag && ndo->ndo_vflag < 3;
}

is_in_line(netdissect_options *ndo, int indent)
{
    return indent - 1 >= ndo->ndo_vflag && ndo->ndo_vflag < 3;
}
