[DI] Fix 'undefined index' error, when entering scope recursively

Imagine two scopes:

```php
$container = new Container();
$container->addScope(new Scope('foo'));
$container->addScope(new Scope('bar', 'foo'));
```

Enter the same scope twice recursively:

```php
$container->enterScope('foo');
// not entering bar in between
$container->enterScope('foo');
// prints warning: undefined index: bar
// at Symfony/Component/DependencyInjection/Container.php:341
```