Fixes: runtime error: shift exponent -7 is negative
Fixes: 3902/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_EXR_fuzzer-6081926122176512

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 dd8351b1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>