OSV-2020-626 UNKNOWN READ in ot::Buffer::GetNextBuffer

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13412 Crash type: UNKNOWN READ Crash state: ot::Buffer::GetNextBuffer ot::MessagePool::FreeBuffers ot::Coap::ResponsesQueue::DequeueAllResponses...