OSV-2020-1668 Memcpy-param-overlap in ot::Message::Write

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=25226 Crash type: Memcpy-param-overlap Crash state: ot::Message::Write ot::Message::CopyTo ot::Ip6::Ip6::InsertMplOption...