Fixes: left shift of 1073741824 by 1 places cannot be represented in type 'int32_t' (aka 'int')
Fixes: 21245/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5683334274613248

Change to int16_t suggested by Lynne

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>