Fix ejabberd_captcha's listener parsing

If the `captcha_host` is specified without `http://` or `https://` prefix (as [in the sample configuration file][1]), `ejabberd_captcha` tries to figure out the protocol automatically.  This commit fixes the code that does the necessary parsing of the listener configuration.

Thanks go to Leonard for reporting the issue in the conference room.

[1]: https://github.com/processone/ejabberd/blob/cc958f7787a824f9d541a05fdbd37f4c3047ee7d/ejabberd.yml.example#L541