Fixes: left shift of 1 by 63 places cannot be represented in type 'long long'
Fixes: out of array access
Fixes: 7284/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AC3_fuzzer-5767914968842240

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>