[HttpKernel] Fix a possible BC break when overriding Client::setServerParameters() with getContainer() usage

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

In my tests, I'm overriding the `test.client.class` parameter to use a custom class which overrides the `setServerParameters` method.

Inside this method, I use the `$this->getContainer()` method to access some parameters of my application.
Since https://github.com/symfony/symfony/commit/111ac18232dd660a3d23d9d0f1935aa914ffc43c, I get the following error : 

```
PHP Fatal error:  Call to a member function getContainer() on a non-object in /home/gnutix/lamp/liip/jlc/sf2/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Clie  
  nt.php on line 48
```

This PR intends to fix this error.