asfdec: ignore too tiny indexes

Fixes Ticket1521
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/asfdec.c

@@ -1362,7 +1362,7 @@ static void asf_build_simple_index(AVFormatContext *s, int stream_index)
             last_pos=pos;
             }
         }
-        asf->index_read= ict > 0;
+        asf->index_read= ict > 1;
     }
     avio_seek(s->pb, current_pos, SEEK_SET);
 }