Symfony\Component\Console\Style\SymfonyStyle::ask fails formatting output correctly if last character of $question or $default is an escaped Backslash

``` php
$io = new SymfonyStyle($input, $output);
$io->ask('question with default value Backslash\\', '\\');
```

This code snippet will print the following in command line:

```
 question with default value Backslash</info> [</comment>]:
 > 
```
- symfony/console v3.1.3
- php v7.0.9
- Mac OSX v10.11.6
