Validate Doorkeeper PreAuthorization to ensure the client is accessible

As of doorkeeper-gem/doorkeeper#1296 Doorkeeper's
PreAuthorization controller is no longer passed the Client upon instantiation,
but instead retrieves it during validation.

To ensure that the we can access the application via the client, we explicitly
call pre_auth.authorizable in order to invoke `validate_client`
https://github.com/linhdangduy/doorkeeper/blob/a3458b00d89fb79e0f0a4efc733124c3bfe56db6/lib/doorkeeper/oauth/pre_authorization.rb#L71

app/controllers/signin_required_authorizations_controller.rb

@@ -3,7 +3,7 @@ class SigninRequiredAuthorizationsController < Doorkeeper::AuthorizationsControl
   EXPECTED_DOORKEEPER_VERSION = "5.2.0.rc3".freeze
 
   def new
-    if pre_auth.authorizable?
+    if pre_authorizable?
       if skip_authorization? || matching_token?
         if user_has_signin_permission_to_application?
           auth = authorization.authorize
@@ -31,6 +31,10 @@ def create
 
 private
 
+  def pre_authorizable?
+    @pre_authorizable ||= pre_auth.authorizable?
+  end
+
   def user_has_signin_permission_to_application?
     return false if application.nil?
     return false if current_resource_owner.nil?
@@ -39,6 +43,7 @@ def user_has_signin_permission_to_application?
   end
 
   def application
+    pre_authorizable? #Doorkeeper PreAuthorization controller must be validated in-order for the client to be instantiated.
     pre_auth.try(:client).try(:application)
   end
 end