avformat/mlvdec: fail reading a packet with 0 streams

Fixes: NULL pointer dereference
Fixes: 22604/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5667739074297856.fuzz

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

libavformat/mlvdec.c

@@ -393,10 +393,14 @@ static int read_packet(AVFormatContext *avctx, AVPacket *pkt)
 {
     MlvContext *mlv = avctx->priv_data;
     AVIOContext *pb;
-    AVStream *st = avctx->streams[mlv->stream_index];
+    AVStream *st;
     int index, ret;
     unsigned int size, space;
 
+    if (!avctx->nb_streams)
+        return AVERROR_EOF;
+
+    st = avctx->streams[mlv->stream_index];
     if (mlv->pts >= st->duration)
         return AVERROR_EOF;