OSV-2020-690 UNKNOWN READ in ot::Coap::CoapBase::RemoveResource

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13369 Crash type: UNKNOWN READ Crash state: ot::Coap::CoapBase::RemoveResource ot::MeshCoP::BorderAgent::Stop ot::Mle::Mle::SetStateDetached...