OSV-2020-608 UNKNOWN READ in ot::Ip6::Address::PrefixMatch

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13349 Crash type: UNKNOWN READ Crash state: ot::Ip6::Address::PrefixMatch ot::Dhcp6::Dhcp6Server::RemovePrefixAgent ot::Dhcp6::Dhcp6Server::UpdateService...