avcodec/mpeg4videodec: Move decode_studiovisualobject() parsing in the branch for visual object parsing

Fixes: runtime error: shift exponent -1 is negative
Fixes: 7510/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-5024523356209152

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