fix a bug when root is undefined

src/md5.js

@@ -13,6 +13,8 @@
   var NODE_JS = typeof process == 'object' && process.versions && process.versions.node;
   if (NODE_JS) {
     root = global;
+  }else if (!root){
+    root = {};
   }
   var COMMON_JS = !root.JS_MD5_TEST && typeof module == 'object' && module.exports;
   var AMD = typeof define == 'function' && define.amd;