OSV-2018-55 Heap-buffer-overflow in ot::MeshForwarder::GetFramePriority

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11376 Crash type: Heap-buffer-overflow READ 8 Crash state: ot::MeshForwarder::GetFramePriority ot::MeshForwarder::HandleFragment ot::MeshForwarder::HandleReceivedFrame...