[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 ?

src/Symfony/Component/Security/Http/Firewall/SwitchUserListener.php

@@ -13,6 +13,7 @@ namespace Symfony\Component\Security\Http\Firewall;
 
 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 @@ class SwitchUserListener implements ListenerInterface
             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);

src/Symfony/Component/Security/Tests/Http/Firewall/SwitchUserListenerTest.php

@@ -149,6 +149,54 @@ class SwitchUserListenerTest extends \PHPUnit_Framework_TestCase
         $listener->handle($this->event);
     }
 
+    public function testExitUserDoesNotDispatchEventWithStringUser()
+    {
+        $originalUser = 'anon.';
+        $refreshedUser = $this->getMock('Symfony\Component\Security\Core\User\UserInterface');
+        $this
+            ->userProvider
+            ->expects($this->never())
+            ->method('refreshUser');
+        $originalToken = $this->getToken();
+        $originalToken
+            ->expects($this->any())
+            ->method('getUser')
+            ->willReturn($originalUser);
+        $role = $this
+            ->getMockBuilder('Symfony\Component\Security\Core\Role\SwitchUserRole')
+            ->disableOriginalConstructor()
+            ->getMock();
+        $role
+            ->expects($this->any())
+            ->method('getSource')
+            ->willReturn($originalToken);
+        $this
+            ->securityContext
+            ->expects($this->any())
+            ->method('getToken')
+            ->willReturn($this->getToken(array($role)));
+        $this
+            ->request
+            ->expects($this->any())
+            ->method('get')
+            ->with('_switch_user')
+            ->willReturn('_exit');
+        $this
+            ->request
+            ->expects($this->any())
+            ->method('getUri')
+            ->willReturn('/');
+
+        $dispatcher = $this->getMock('Symfony\Component\EventDispatcher\EventDispatcherInterface');
+        $dispatcher
+            ->expects($this->never())
+            ->method('dispatch')
+        ;
+
+        $listener = new SwitchUserListener($this->securityContext, $this->userProvider, $this->userChecker, 'provider123', $this->accessDecisionManager, null, '_switch_user', 'ROLE_ALLOWED_TO_SWITCH', $dispatcher);
+        $listener->handle($this->event);
+    }
+
     /**
      * @expectedException \Symfony\Component\Security\Core\Exception\AccessDeniedException
      */