Squelch a "const object should be initialized" warning.

Squelch a "const object should be initialized" warning.

MSVC says a const object should be initialized, so initialize it.

Use nd_ipv6 as the type, while we're at it (it shouldn't make a
difference, but this means we know the type - array of 16 octets - so we
can just initialize it with { 0 }).