OSV-2020-597 UNKNOWN READ in ot::MessageQueue::GetHead

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13386 Crash type: UNKNOWN READ Crash state: ot::MessageQueue::GetHead ot::Coap::CoapBase::ClearRequestsAndResponses ot::Coap::CoapSecure::Stop...