avcodec/ituh263dec: Avoid spending a long time in slice sync

Fixes: 177/fuzz-3-ffmpeg_VIDEO_AV_CODEC_ID_FLV1_fuzzer

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