OSV-2020-336 Stack-buffer-overflow in ot::NetworkData::PrefixTlv::Init

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=12589 Crash type: Stack-buffer-overflow READ Crash state: ot::NetworkData::PrefixTlv::Init ot::NetworkData::Local::AddOnMeshPrefix otError ot::Ncp::NcpBase::HandlePropertyInsert...