lavf: ignore attachment streams for interleaving purposes

Those streams should never get any packets by definition.

libavformat/utils.c

@@ -2633,6 +2633,7 @@ void avformat_free_context(AVFormatContext *s)
     av_freep(&s->chapters);
     av_dict_free(&s->metadata);
     av_freep(&s->streams);
+    av_freep(&s->internal);
     av_free(s);
 }