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