Correctly detect PLAT prefixes containing embedded WKAs
Correctly detect PLAT prefixes containing embedded WKAs

The previous code would fail to detect PLAT prefix if ipv4only.arpa resolved to
an address such as 2001:db8:c000:aa::c000:aa. It would get confused as to
whether the prefix was 2001:db8::/32 (incorrect) or 2001:db8:c000:aa::/96
(correct).

Bug reported by Kasper Dupont - thanks!

toreanderson/clatd,toreanderson/clatd