Fixes: store to null pointer of type 'uint32_t' (aka 'unsigned int')
Fixes: 18021/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APTX_HD_fuzzer-5761738313564160

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