OSV-2020-692 Heap-use-after-free in WelsDec::SetUnRef

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=18458 Crash type: Heap-use-after-free WRITE 1 Crash state: WelsDec::SetUnRef WelsDec::SlidingWindow WelsDec::WelsMarkAsRef...