dhcp: NULL pointer dereference crash via crafted DHCPv6 packet

ISC DHCP server 4.0 before 4.0.2, 4.1 before 4.1.2, and 4.2 before 4.2.0-P1 allows remote attackers to cause a denial of service NULL pointer dereference and crash via a DHCPv6 packet containing a Relay-Forward message without an address in the Relay-Forward link-address field...