Fix a fatal error in some PHP versions.

Takes care of:

```
PHP Fatal error:  Cannot create references to elements of a temporary array expression in /var/www/sylius/src/Sylius/Bundle/SearchBundle/Indexer/OrmIndexer.php on line 123

Fatal error: Cannot create references to elements of a temporary array expression in /var/www/sylius/src/Sylius/Bundle/SearchBundle/Indexer/OrmIndexer.php on line 123
```

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -
I think this won't work until you combine the new keys in the fields array:
$fields = array_combine($fieldNames, array_values($fields));