avcodec/gdv: Check compression before allocating frame

Fixes: 2926/clusterfuzz-testcase-4987110014582784

This reduces decoding time from 7 to 4 seconds

The timeout should have been fixed in 0561bd2fc2bff0dbe651d5998e9f129c43d25eb3 but ossfuzz did not close this issue

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