CVE-2017-13042/HNCP: add DHCPv6-Data bounds checks

CVE-2017-13042/HNCP: add DHCPv6-Data bounds checks

hncp_print_rec() validates each HNCP TLV to be within the declared as
well as the on-the-wire packet space. However, dhcpv6_print() in the same
file didn't do the same for the DHCPv6 options within the HNCP
DHCPv6-Data TLV value, which could cause an out-of-bounds read when
decoding an invalid packet. Add missing checks to dhcpv6_print().

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).