asf: only set index_read if the index contained entries.

This allows falling back to a binary search if the file contains no
index, thus fixing seeking in such files (e.g. luckynight.wma).

libavformat/asfdec.c

@@ -1235,7 +1235,7 @@ static void asf_build_simple_index(AVFormatContext *s, int stream_index)
             last_pos=pos;
             }
         }
-        asf->index_read= 1;
+        asf->index_read= ict > 0;
     }
     avio_seek(s->pb, current_pos, SEEK_SET);
 }