[DomCrawler] Fixed a fatal error when setting a value in a malformed field name

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Hi,

I found a case where a fatal error happen when trying to set a value in a form field with a malformed name.

```
Fatal error: Call to a member function setValue() on a non-object in /home/pierreyves/projects/symfony2/symfony/src/Symfony/Component/DomCrawler/FormFieldRegistry.php on line 128

Call Stack:
    0.0001     231832   1. {main}() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/composer/bin/phpunit:0
    0.0035     688768   2. PHPUnit_TextUI_Command::main() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/composer/bin/phpunit:63
    0.0035     689000   3. PHPUnit_TextUI_Command->run() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/TextUI/Command.php:129
    0.0224    2705448   4. PHPUnit_TextUI_TestRunner->doRun() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/TextUI/Command.php:176
    0.0301    2997392   5. PHPUnit_Framework_TestSuite->run() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/TextUI/TestRunner.php:349
    0.0477    3613296   6. PHPUnit_Framework_TestSuite->runTest() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/Framework/TestSuite.php:745
    0.0477    3613296   7. PHPUnit_Framework_TestCase->run() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/Framework/TestSuite.php:775
    0.0477    3613296   8. PHPUnit_Framework_TestResult->run() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php:776
    0.0478    3614208   9. PHPUnit_Framework_TestCase->runBare() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/Framework/TestResult.php:648
    0.0478    3630992  10. PHPUnit_Framework_TestCase->runTest() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php:831
    0.0478    3631856  11. ReflectionMethod->invokeArgs() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php:976
    0.0478    3631888  12. Symfony\Component\DomCrawler\Tests\FormTest->testSetValueOnMultiValuedFieldsWithMalformedName() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php:976
    0.0481    3635560  13. Symfony\Component\DomCrawler\Form->offsetSet() /home/pierreyves/projects/symfony2/symfony/vendor/phpunit/phpunit/PHPUnit/Framework/TestCase.php:255
    0.0481    3635560  14. Symfony\Component\DomCrawler\FormFieldRegistry->set() /home/pierreyves/projects/symfony2/symfony/src/Symfony/Component/DomCrawler/Form.php:320
```

In this case, `FormFieldRegistry::getSegments('foo[bar')` method is called and it will return `array('foo')` only. Therefore the return of `FormFieldRegistry::get('foo[bar')` returns an array instead of a `FormField` and so the `setValue()` call happen on the array which leads to the fatal error.

I tried to fix that. I don't know if it's the best way to do this so, as always, comments are welcome.