net: ipv6: Make sure we do not access link address past array length

It is possible to manually set link address length past 6 at runtime
and trying to generate IPv6 IID address that way. This should fail
as we could read two bytes past the address buffer. There is no issues
in the copying as the target buffer has plenty of space.

Coverity-CID: 516232

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>