Fixes: null pointer dereference
Fixes: 8946/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DPX_fuzzer-5078915222601728

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