bug #23581 [Config] Minor fix (nicolas-grekas)

This PR was merged into the 3.3 branch.

Discussion
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[Config] Minor fix

| Q             | A
| ------------- | ---
| Branch?       | 3.3
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

My bad.

BTW @javiereguiluz I confirm my diagnosis on #23573

Here is the last exception in that error page:
![capture du 2017-07-19 09-37-41](https://user-images.githubusercontent.com/243674/28355877-2bc71b5e-6c66-11e7-8e53-a88c42bec4e5.png)

Commits
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fa0b942 [Config] Minor fix

src/Symfony/Component/Config/Resource/ClassExistenceResource.php

@@ -149,7 +149,7 @@ class ClassExistenceResource implements SelfCheckingResourceInterface, \Serializ
             $props = array(
                 'file' => $trace[$i]['file'],
                 'line' => $trace[$i]['line'],
-                'trace' => array_slice($trace, 0, 1 + $i),
+                'trace' => array_slice($trace, 1 + $i),
             );
 
             foreach ($props as $p => $v) {