Fixes error with unperisted dynamic route in sonata FrontendLinkExtension

Fixes a serious error at the ```\Symfony\Cmf\Bundle\RoutingBundle\Admin\Extension\FrontendLinkExtension``` when a new dynamic route is created in sonata admin:

```
An exception has been thrown during the rendering of a template ("Can not determine the prefix. 
Either this is a new, unpersisted document or the listener that calls setPrefix is not set up correctly.") 
in SonataAdminBundle:CRUD:base_edit.html.twig at line 34.
```

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony-cmf/cmf-sandbox#276
| License       | MIT
| Doc PR        | none
The link is already being tested (```/cmf/routing/route/create```). Trying to get it tested somehow...