Fixes: runtime error: left shift of negative value -801112064
Fixes: 3492/clusterfuzz-testcase-minimized-5784775283441664

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 fca198fb)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>