avformat/img2enc: fix writing multiple streams in write_muxed_file

Maybe we should just reject multiple streams for the image2 muxer instead?
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/img2enc.c

@@ -74,7 +74,7 @@ static int write_header(AVFormatContext *s)
 static int write_muxed_file(AVFormatContext *s, AVIOContext *pb, AVPacket *pkt)
 {
     VideoMuxData *img = s->priv_data;
-    AVCodecParameters *par = s->streams[0]->codecpar;
+    AVCodecParameters *par = s->streams[pkt->stream_index]->codecpar;
     AVStream *st;
     AVPacket pkt2 = {0};
     AVFormatContext *fmt = NULL;
@@ -93,12 +93,17 @@ static int write_muxed_file(AVFormatContext *s, AVIOContext *pb, AVPacket *pkt)
 
     fmt->pb = pb;
 
-    if ((ret = av_packet_ref(&pkt2, pkt))                      < 0 ||
-        (ret = avcodec_parameters_copy(st->codecpar, par))     < 0 ||
+    ret = av_packet_ref(&pkt2, pkt);
+    if (ret < 0)
+        goto out;
+    pkt2.stream_index = 0;
+
+    if ((ret = avcodec_parameters_copy(st->codecpar, par))     < 0 ||
         (ret = avformat_write_header(fmt, NULL))               < 0 ||
         (ret = av_interleaved_write_frame(fmt, &pkt2))         < 0 ||
         (ret = av_write_trailer(fmt))) {}
 
+out:
     av_packet_unref(&pkt2);
     avformat_free_context(fmt);
     return ret;