Fixed a bug in the Categories hierarchy. (#48, #57)

When one post have only one category will hit the bug.

tabs/categories.md

@@ -20,9 +20,11 @@ title: Categories
 
     {% for post in posts_of_category %}
       {% assign second_category = post.categories[1] %}
+      {% if second_category %}
         {% unless sub_categories contains second_category %}
           {% assign sub_categories = sub_categories | push: second_category %}
         {% endunless %}
+      {% endif %}
     {% endfor %}
 
     {% assign sub_categories = sub_categories | sort %}