Fixes: runtime error: shift exponent -1 is negative
Fixes: 3091/clusterfuzz-testcase-minimized-6229767969832960

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 2a83866c)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>