avcodec/mpeg4videodec: Fix GMC with videos of dimension 1

Fixes: runtime error: shift exponent -1 is negative
Fixes: 2338/clusterfuzz-testcase-minimized-5153426541379584

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
