Fix undefined function drush_get_user_id() error

If I run `drush @example.local runserver` I get this error:

`PHP Fatal error:  Call to undefined function drush_get_user_id() in /home/kosta/src/drupal/drush/commands/runserver/runserver.drush.inc on line 202`

This patch checks if the user has passed the `--user` flag (and therefore bootstrapped to DRUPAL_LOGIN phase) before trying to use `drush_get_user_id()`.