OSV-2020-681 Heap-use-after-free in WelsDec::SetUnRef

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=18459 Crash type: Heap-use-after-free WRITE 1 Crash state: WelsDec::SetUnRef WelsDec::WelsDelLongFromListSetUnref WelsDec::MMCOProcess...