Fixes: shift exponent -1 is negative
Fixes: 63889/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6009343056936960

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