test-dhcp6-client: add one more assert on memory mapping

test-dhcp6-client: add one more assert on memory mapping

Static analyzers need a hint that optval is not pointing
off the end of the msg_advertise array, since pos can go
up to the full length of it. The array is manually
constructed so we know this won't happen, but adding one
more assert should be enough to avoid false positives.

Coverity CID #1394277