Ensure #route_setting is available before calling it

In the context of an exception, #route_setting is not available.

See:

- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14178#note_40259442
- https://github.com/ruby-grape/grape/blob/df79bc35e43f0d5b6cbd666b587cf4640de174b8/lib/grape/middleware/error.rb#L59Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased-ee/sh-fix-api-exception-handling.yml

+---
+title: 'Ensure #route_setting is available before calling it'
+merge_request: 2908
+author:
+type: fixed

lib/api/api_guard.rb

@@ -106,6 +106,8 @@ module API
       private
 
       def route_authentication_setting
+        return {} unless respond_to?(:route_setting)
+
         route_setting(:authentication) || {}
       end