[Yaml] Regression in object support parser fix

The fix for #17075 done in #17079 introduced a regression.

The following example:

``` php
use Symfony\Component\Yaml\Yaml;

print_r(Yaml::parse("foo: bar\nbaz: derp", false, false, true));
```

Is failing with the following exception:

```
PHP Fatal error:  Cannot use object of type stdClass as array in /path/to/vendor/symfony/symfony/src/Symfony/Component/Yaml/Parser.php on line 236
```
