Fixes: index 20 out of bounds for type 'const char *[4][128]'
Fixes: 14367/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CCAPTION_fuzzer-5718819672162304
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>