OSV-2020-2121 Use-of-uninitialized-value in WelsDec::BaseMC

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=26861 Crash type: Use-of-uninitialized-value Crash state: WelsDec::BaseMC WelsDec::GetInterBPred WelsDec::WelsMbInterPrediction...