fix OutOfBoundsException when using mail or sendmail transport

Hi,

Since the symfony/SwiftmailerBundle#34 had been merged, an exception occured when using mail or sendmail transport : 

```
[Symfony\Component\DependencyInjection\Exception\OutOfBoundsException]  
  The index "1" is not in the range [0, 0]. 
```

This patch fix it.