avcodec/gdv: Fix undefined shift

Fixes: runtime error: left shift of 255 by 24 places cannot be represented in type 'int'
Fixes: 2249/clusterfuzz-testcase-minimized-5388542379294720

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