OSV-2020-721 Stack-buffer-overflow in ot::NetworkData::NetworkData::GetNextOnMeshPrefix

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13334 Crash type: Stack-buffer-overflow WRITE Crash state: ot::NetworkData::NetworkData::GetNextOnMeshPrefix ot::Dhcp6::Dhcp6Client::UpdateAddresses ot::Mle::Mle::HandleStateChanged...