avcodec/movtextdec: Fix tsmb_size check==0 check

Fixes: 173/fuzz-3-ffmpeg_SUBTITLE_AV_CODEC_ID_MOV_TEXT_fuzzer

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