avcodec/mpeg4videodec: Fix undefined shift in get_amv()
Fixes: runtime error: shift exponent -1 is negative
Fixes: 9938/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-5653783529914368
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c88afa44c4
)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>