bug #19334 [Security] Fix the retrieval of the last username when using forwarding (stof)

This PR was merged into the 2.7 branch.

Discussion
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[Security] Fix the retrieval of the last username when using forwarding

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

When using forwarding to render the login page (which is not the default), the info are stored in the subrequest attributes rather than the session. ``getLastAuthenticationError`` was handling this properly but ``getLastUsername`` was not checking the attributes.
This fixes it by checking the attributes (I'm checking them before the session, to be consistent with ``getLastAuthenticationError``)

Commits
-------

e041365 Fix the retrieval of the last username when using forwarding

src/Symfony/Component/Security/Http/Authentication/AuthenticationUtils.php

@@ -65,7 +65,13 @@ class AuthenticationUtils
      */
     public function getLastUsername()
     {
-        $session = $this->getRequest()->getSession();
+        $request = $this->getRequest();
+
+        if ($request->attributes->has(Security::LAST_USERNAME)) {
+            return $request->attributes->get(Security::LAST_USERNAME);
+        }
+
+        $session = $request->getSession();
 
         return null === $session ? '' : $session->get(Security::LAST_USERNAME);
     }