minor #5580 Additional User check in voter class (weaverryan)

This PR was merged into the 2.3 branch.

Discussion
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Additional User check in voter class

Finishes #5317

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #5279

Commits
-------

9ad9daf Additional little check to show how we're assumign the User object is the User entity

cookbook/security/voters.rst

@@ -77,6 +77,7 @@ edit a particular object. Here's an example implementation::
     // src/AppBundle/Security/Authorization/Voter/PostVoter.php
     namespace AppBundle\Security\Authorization\Voter;
 
+    use AppBundle\Entity\User;
     use Symfony\Component\Security\Core\Authorization\Voter\VoterInterface;
     use Symfony\Component\Security\Core\Authentication\Token\TokenInterface;
     use Symfony\Component\Security\Core\User\UserInterface;
@@ -133,6 +134,13 @@ edit a particular object. Here's an example implementation::
                 return VoterInterface::ACCESS_DENIED;
             }
 
+            // double-check that the User object is the expected entity.
+            // It always will be, unless there is some misconfiguration of the
+            // security system.
+            if (!$user instanceof User) {
+                throw new \LogicException('The user is somehow not our User class!');
+            }
+
             switch($attribute) {
                 case self::VIEW:
                     // the data object could have for example a method isPrivate()