OSV-2020-1292 Stack-buffer-overflow in ot::ExtendedTlv::GetLength

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=20775 Crash type: Stack-buffer-overflow READ 2 Crash state: ot::ExtendedTlv::GetLength ot::Tlv::GetSize ot::Tlv::GetNext...