avformat/vividas: Fix n_sb_blocks Check

Fixes: signed integer overflow: 1540265776 * 2 cannot be represented in type 'int'
Fixes: 18160/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5758808818712576

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

libavformat/vividas.c

@@ -438,7 +438,7 @@ static int track_index(VividasDemuxContext *viv, AVFormatContext *s, uint8_t *bu
     ffio_read_varlen(pb); // track_index_len
     avio_r8(pb); // 'c'
     viv->n_sb_blocks = ffio_read_varlen(pb);
-    if (viv->n_sb_blocks * 2 > size)
+    if (viv->n_sb_blocks < 0 || viv->n_sb_blocks > size / 2)
         goto error;
     viv->sb_blocks = av_calloc(viv->n_sb_blocks, sizeof(VIV_SB_block));
     if (!viv->sb_blocks) {