CVE-2017-13017/DHCPv6: Add a missing option length check.

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), modified
so the capture file won't be rejected as an invalid capture.
dhcp6stcode(const uint16_t code)
{
	return code > 255 ? "INVALID code" : tok2str(dh6opt_stcode_str, "code%u", code);
}

dhcp6stcode(const uint16_t code)
{
	return code > 255 ? "INVALID code" : tok2str(dh6opt_stcode_str, "code%u", code);
}
