avformat/vividas: Fix crash when seeking without audio stream

The current code tries the access the codecpar of a nonexistent
audio stream when seeking. Stop that. Fixes ticket #9121.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
(cherry picked from commit af867e59d9)

libavformat/vividas.c

@@ -757,18 +757,23 @@ static int viv_read_seek(AVFormatContext *s, int stream_index, int64_t timestamp
 
     for (int i = 0; i < viv->n_sb_blocks; i++) {
         if (frame >= viv->sb_blocks[i].packet_offset && frame < viv->sb_blocks[i].packet_offset + viv->sb_blocks[i].n_packets) {
-            // flush audio packet queue
-            viv->current_audio_subpacket = 0;
-            viv->n_audio_subpackets = 0;
             viv->current_sb = i;
             // seek to ith sb block
             avio_seek(s->pb, viv->sb_offset + viv->sb_blocks[i].byte_offset, SEEK_SET);
             // load the block
             load_sb_block(s, viv, 0);
-            // most problematic part: guess audio offset
-            viv->audio_sample = av_rescale_q(viv->sb_blocks[i].packet_offset, av_make_q(s->streams[1]->codecpar->sample_rate, 1), av_inv_q(s->streams[0]->time_base));
-            // hand-tuned 1.s a/v offset
-            viv->audio_sample += s->streams[1]->codecpar->sample_rate;
+            if (viv->num_audio) {
+                const AVCodecParameters *par = s->streams[1]->codecpar;
+                // flush audio packet queue
+                viv->current_audio_subpacket = 0;
+                viv->n_audio_subpackets      = 0;
+                // most problematic part: guess audio offset
+                viv->audio_sample = av_rescale_q(viv->sb_blocks[i].packet_offset,
+                                                 av_make_q(par->sample_rate, 1),
+                                                 av_inv_q(s->streams[0]->time_base));
+                // hand-tuned 1.s a/v offset
+                viv->audio_sample += par->sample_rate;
+            }
             viv->current_sb_entry = 0;
             return 1;
         }