set featured_id to 0 if the featured image is deleted

ext/featured/main.php

@@ -71,6 +71,15 @@ class Featured extends Extension
         }
     }
 
+    public function onImageDeletion(ImageDeletionEvent $event)
+    {
+        global $config;
+        if($event->image->id == $config->get_int("featured_id")) {
+            $config->set_int("featured_id", 0);
+            $config->save();
+        }
+    }
+
     public function onImageAdminBlockBuilding(ImageAdminBlockBuildingEvent $event)
     {
         global $user;