sd-dhcp6-client: Fix unreferencing DHCPv6 lease on client reset

sd-dhcp6-client: Fix unreferencing DHCPv6 lease on client reset

When the DHCPv6 client is started by the library user or stopped for
any reason, unref the DHCPv6 lease when resetting the DHCPv6 client
data structure. This makes the DHCPv6 client always start from a clean
state and not keep unnecessary an lease structure around when stopped.

If this is not done, a previously existing lease information can be
interpreted to be from another server when restarting DHCPv6.