sessions: fix error when an anonymous user tried to logout.

Fix an exception when a user who was already logged out tried to logout
again.

app/logical/session_loader.rb

@@ -31,6 +31,7 @@ class SessionLoader
 
   def logout
     session.delete(:user_id)
+    return if CurrentUser.user.is_anonymous?
     UserEvent.create_from_request!(CurrentUser.user, :logout, request)
   end
 

test/functional/sessions_controller_test.rb

@@ -75,18 +75,25 @@ class SessionsControllerTest < ActionDispatch::IntegrationTest
     end
 
     context "destroy action" do
-      setup do
-        delete_auth session_path, @user
-      end
-
       should "clear the session" do
+        delete_auth session_path, @user
+
         assert_redirected_to posts_path
         assert_nil(session[:user_id])
       end
 
       should "generate a logout event" do
+        delete_auth session_path, @user
+
         assert_equal(true, @user.user_events.logout.exists?)
       end
+
+      should "not fail if the user is already logged out" do
+        delete session_path
+
+        assert_redirected_to posts_path
+        assert_nil(session[:user_id])
+      end
     end
 
     context "sign_out action" do