OSV-2020-619 UNKNOWN READ in ot::Ip6::Netif::UnsubscribeAllRoutersMulticast

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=13353 Crash type: UNKNOWN READ Crash state: ot::Ip6::Netif::UnsubscribeAllRoutersMulticast ot::Mle::MleRouter::HandleDetachStart ot::Mle::Mle::SetStateDetached...