fix bug wordcount (#654)

includes/helpers/site.js

@@ -57,6 +57,9 @@ module.exports = function (hexo) {
      * Get the word count of a paragraph.
      */
     hexo.extend.helper.register('word_count', function (content) {
+	if (typeof(content) == "undefined") {
+            return 0
+        }
         content = content.replace(/<\/?[a-z][^>]*>/gi, '');
         content = content.trim();
         return content ? (content.match(/[\u00ff-\uffff]|[a-zA-Z]+/g) || []).length : 0;
@@ -103,4 +106,4 @@ module.exports = function (hexo) {
     hexo.extend.helper.register('hexo_version', function (data) {
         return hexo.version;
     });
-}
+}