Fixes: left shift of negative value -14614752
Fixes: 15174/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5670543606415360

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>