avformat/lvfdec: Check stream_index before use

Fixes: assertion failure
Fixes: 26905/clusterfuzz-testcase-minimized-ffmpeg_dem_LVF_fuzzer-5724267599364096.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/lvfdec.c

@@ -106,6 +106,7 @@ static int lvf_read_packet(AVFormatContext *s, AVPacket *pkt)
     unsigned size, flags, timestamp, id;
     int64_t pos;
     int ret, is_video = 0;
+    int stream_index;
 
     pos = avio_tell(s->pb);
     while (!avio_feof(s->pb)) {
@@ -121,12 +122,15 @@ static int lvf_read_packet(AVFormatContext *s, AVPacket *pkt)
         case MKTAG('0', '1', 'w', 'b'):
             if (size < 8)
                 return AVERROR_INVALIDDATA;
+            stream_index = is_video ? 0 : 1;
+            if (stream_index >= s->nb_streams)
+                return AVERROR_INVALIDDATA;
             timestamp = avio_rl32(s->pb);
             flags = avio_rl32(s->pb);
             ret = av_get_packet(s->pb, pkt, size - 8);
             if (flags & (1 << 12))
                 pkt->flags |= AV_PKT_FLAG_KEY;
-            pkt->stream_index = is_video ? 0 : 1;
+            pkt->stream_index = stream_index;
             pkt->pts          = timestamp;
             pkt->pos          = pos;
             return ret;