avcodec/ulti: Check number of blocks at init

Fixes: Timeout
Fixes: 4832/clusterfuzz-testcase-4699096590843904

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

libavcodec/ulti.c

@@ -50,6 +50,8 @@ static av_cold int ulti_decode_init(AVCodecContext *avctx)
     s->width = avctx->width;
     s->height = avctx->height;
     s->blocks = (s->width / 8) * (s->height / 8);
+    if (s->blocks == 0)
+        return AVERROR_INVALIDDATA;
     avctx->pix_fmt = AV_PIX_FMT_YUV410P;
     s->ulti_codebook = ulti_codebook;