Fix issue if "most recent comment" not found

Not sure exactly how this can happen, but I've seen a few errors caused
by this.

tildes/tildes/templates/topic.jinja2

@@ -323,12 +323,14 @@
       ({{ pluralize(comments.num_top_level, "thread") }})
     </dd>
 
+    {% if comments.most_recent_comment %}
       <dt>Last comment posted</dt>
       <dd>
         <a href="{{ comments.most_recent_comment.permalink }}" data-js-hide-sidebar-no-preventdefault>
           {{ adaptive_date_responsive(comments.most_recent_comment.created_time) }}
         </a>
       </dd>
+    {% endif %}
   {% else %}
     <dd>No comments yet</dd>
   {% endif %}