Fixes: left shift of negative value -2
Fixes: 20305/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUEHD_fuzzer-5677196618498048

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Jai Luthra <me@jailuthra.in>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>