[FrameworkBundle] fixed typo

src/Symfony/Bundle/FrameworkBundle/Resources/views/Exception/exception.html.twig

@@ -20,7 +20,7 @@
                 </div>
 
                 {% set previous_count = exception.allPrevious|length %}
-                {% if previous_count|length %}
+                {% if previous_count %}
                     <div class="linked"><span><strong>{{ previous_count }}</strong> linked Exception{{ previous_count > 1 ? 's' : '' }}:</span>
                         <ul>
                             {% for i, previous in exception.allPrevious %}