Fix the uninitialized use of source IPv6 address in NAT64LSN.

This code is already refactored in head/, but due to the missing
epoch(9) support it is impossible to merge. So, it is direct commit to
stable/11.

Reported by: Patrick M. Hausen <hausen punkt de>
Tested by: Patrick M. Hausen <hausen punkt de>
MFC after: 3 days