sd-dhcp6-lease: ignore invalid byte(s) at the end of the packet

sd-dhcp6-lease: ignore invalid byte(s) at the end of the packet

Oracle Cloud sends malformed DHCPv6 replies that have an invalid
byte at the end, which cannot be parsed as an option code.

networkd currently can cope with the invalid option (it is ignored),
but the whole packet is ignored altogether because of the additional
null at the end.

It's better to be liberal in what we accept and actually assign an
address, given that the reply contains a valid IA_NA.

Fixes #28183.