Fixes: runtime error: left shift of negative value -127
Fixes: 2834/clusterfuzz-testcase-minimized-5988039123795968

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 62702eeb)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>