fix user result check on user controller

includes/controller/users_controller.php

@@ -81,6 +81,12 @@ function user_controller() {
   
   if (isset($_REQUEST['user_id'])) {
     $user_source = User($_REQUEST['user_id']);
+    if ($user_source === false)
+      engelsystem_error("Unable to load user.");
+    if ($user_source == null) {
+      error(_("User not found."));
+      redirect('?');
+    }
   } else
     $user_source = $user;