avformat/vividas: Check and require 1 video stream

The decoder hardcodes that audio is stream_id = 1 so it does not
currently work with more or less than 1 video stream at st=0

Fixes: assertion failure
Fixes: 18602/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6259277199310848

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

@@ -311,8 +311,10 @@ static int track_header(VividasDemuxContext *viv, AVFormatContext *s,  uint8_t *
     num_video = avio_r8(pb);
 
     avio_seek(pb, off, SEEK_SET);
-    if (num_video != 1)
-        av_log(s, AV_LOG_WARNING, "number of video tracks %d is not 1\n", num_video);
+    if (num_video != 1) {
+        av_log(s, AV_LOG_ERROR, "number of video tracks %d is not 1\n", num_video);
+        return AVERROR_PATCHWELCOME;
+    }
 
     for (i = 0; i < num_video; i++) {
         AVStream *st = avformat_new_stream(s, NULL);