net: ipv6: Do not access link address past array length

It is possible to manually set link address length past 6 at runtime
and trying to check IPv6 ll address that way. This should fail
as we could read two bytes past the address buffer.

Coverity-CID: 516242

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