lavf/concat: reject empty scripts.

Fix trac ticket #2566.

libavformat/concatdec.c

@@ -216,6 +216,8 @@ static int concat_read_header(AVFormatContext *avf)
     }
     if (ret < 0)
         FAIL(ret);
+    if (!cat->nb_files)
+        FAIL(AVERROR_INVALIDDATA);
 
     for (i = 0; i < cat->nb_files; i++) {
         if (cat->files[i].start_time == AV_NOPTS_VALUE)