avcodec/cdxl: Check format for BGR24

      Fixes: out of array access
Fixes: 1427/clusterfuzz-testcase-minimized-5020737339392000

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