Fixes: left shift of negative value -1
Fixes: 4690/clusterfuzz-testcase-minimized-6117482428366848

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit d135f3c5)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>