OSV-2020-596 UNKNOWN READ in ot::PriorityQueue::GetHead

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13388 Crash type: UNKNOWN READ Crash state: ot::PriorityQueue::GetHead ot::Ip6::Ip6::HandleSendQueue ot::TaskletScheduler::ProcessQueuedTasklets...