OSV-2020-2045 Use-of-uninitialized-value in WelsDec::GetColocatedMb

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=26272 Crash type: Use-of-uninitialized-value Crash state: WelsDec::GetColocatedMb WelsDec::PredBDirectTemporal WelsDec::WelsDecodeMbCavlcBSlice...