Prevent Breadcrumbs on 404

This fixes the PHP error `Undefined array key 404`. Thought about adding a specific key for that case, but as the content is not found it is obsolete IMO.

src/Crumb.php

@@ -63,7 +63,7 @@ protected function add($key, $value = null, $blog = false)
      */
     public function build()
     {
-        if (is_front_page()) {
+        if (is_front_page() || is_404()) {
             return $this->breadcrumb;
         }