Fix undefined variable

src/Wallabag/CoreBundle/Resources/views/Entry/_feed_link.html.twig

@@ -9,6 +9,6 @@
     {% set feed_route = feed_route ~ '_feed' %}
     {% set slug = null %}
 {% endif %}
-{% if feed_route %}
+{% if feed_route is defined %}
     <a rel="alternate" type="application/atom+xml" href="{{ path(feed_route, {'username': app.user.username, 'token': app.user.config.feedToken, 'slug': slug}) }}" class="results-item"><i class="material-icons">rss_feed</i></a>
 {% endif %}