OSV-2020-2264 Stack-buffer-overflow in ot::IsValidUtf8String

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28682 Crash type: Stack-buffer-overflow READ 1 Crash state: ot::IsValidUtf8String ot::MeshCoP::NetworkNameTlv::IsValid ot::MeshCoP::Tlv::IsValid...