CVE-2017-13041/ICMP6: Add more bounds checks.

This fixes a buffer over-read discovered by Kim Gwan Yeong.

Add a test using the capture file supplied by the reporter(s).
static int icmp6_cksum(netdissect_options *ndo, const struct ip6_hdr *ip6,
	const struct icmp6_hdr *icp, u_int len)
{
	return nextproto6_cksum(ndo, ip6, (const uint8_t *)(const void *)icp, len, len,
				IPPROTO_ICMPV6);
}

static int icmp6_cksum(netdissect_options *ndo, const struct ip6_hdr *ip6,
	const struct icmp6_hdr *icp, u_int len)
{
	return nextproto6_cksum(ndo, ip6, (const uint8_t *)(const void *)icp, len, len,
				IPPROTO_ICMPV6);
}
