avcodec/htmlsubtitles: Fix reading one byte beyond the array

Fixes: fuzz-2-ffmpeg_SUBTITLE_AV_CODEC_ID_SUBRIP_fuzzer

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