asfdec: set nb_streams to 0 in the asf_read_close

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/asfdec.c

@@ -1457,6 +1457,7 @@ static int asf_read_close(AVFormatContext *s)
         av_dict_free(&asf->asf_sd[i].asf_met);
     }
 
+    asf->nb_streams = 0;
     return 0;
 }