[Security] Fixed SwitchUserListener when exiting an impersonication with AnonymousToken

If you configure a firewall with switch user with `role: IS_AUTHENTICATED_ANONYMOUSLY` it's impossible to exit the
impersonation because the next line `$this->provider->refreshUser($original->getUser())` will fail. It fails because `RefreshUser`
expects an instance of `UserInterface` and here it's a string.

Therefore, it does not make sense to refresh an Anonymous Token, right ?

Author: lyrixx

Firewall/SwitchUserListener.php

@@ -13,6 +13,7 @@
 
 use Symfony\Component\Security\Core\Exception\AccessDeniedException;
 use Symfony\Component\Security\Core\SecurityContextInterface;
+use Symfony\Component\Security\Core\User\UserInterface;
 use Symfony\Component\Security\Core\User\UserProviderInterface;
 use Symfony\Component\Security\Core\User\UserCheckerInterface;
 use Symfony\Component\Security\Core\Authorization\AccessDecisionManagerInterface;
@@ -162,7 +163,7 @@ private function attemptExitUser(Request $request)
             throw new AuthenticationCredentialsNotFoundException('Could not find original Token object.');
         }
 
-        if (null !== $this->dispatcher) {
+        if (null !== $this->dispatcher && $original->getUser() instanceof UserInterface) {
             $user = $this->provider->refreshUser($original->getUser());
             $switchEvent = new SwitchUserEvent($request, $user);
             $this->dispatcher->dispatch(SecurityEvents::SWITCH_USER, $switchEvent);