FormHelper - Check DateTime instances to avoid fatal errors

Passing an invalid time value to a method that creates a DateTime object using said value causes a fatal error.

```php
$this->request->data['Test']['minute'] = '12a';
echo $this->Form->create('Test');
echo $this->Form->minute('minute');
echo $this->Form->end('Submit');

Error: Call to a member function format() on a non-object
File: /var/www/sandbox/CakePHP/lib/Cake/View/Helper/FormHelper.php
Line: 2281
```