修复 某些情况下默认保存目录不出现在下载目录列表中

bug来源:
transmission.downloadDirs会在system.js中初始化，并保存默认保存目录
种子开始加载时，transmission.downloadDirs又会在transmission.torrents.js重新初始化从而丢失默认保存目录

src/tr-web-control/script/transmission.torrents.js

@@ -91,7 +91,9 @@ transmission.torrents = {
 		this.warning = new Array();
 		this.btItems = new Array();
 		// All download directories used by current torrents
+		if (transmission.downloadDirs == undefined){
 			transmission.downloadDirs = new Array();
+		}
 
 		var _Status = transmission._status;
 		this.status = {};