refactor(fragment_cache): reuse result (hexojs#3985)

Author: SukkaW

lib/plugins/helper/fragment_cache.js

@@ -9,8 +9,9 @@ module.exports = ctx => {
   return function fragmentCache(id, fn) {
     if (this.cache && cache[id] != null) return cache[id];
 
-    cache[id] = fn();
     const result = fn();
+
+    cache[id] = result;
     return result;
   };
 };