Returns the next_url path if next_url matches allowed hosts.

def validate_redirect_url(next_url):
        """
        Returns the next_url path if next_url matches allowed hosts.
        """
        if not next_url:
            return None
        parts = urlparse(next_url)
        if parts.netloc:
            domain, _ = split_domain_port(parts.netloc)
            allowed_hosts = (['*'] if django_settings.DEBUG
                else django_settings.ALLOWED_HOSTS)
            if not (domain and validate_host(domain, allowed_hosts)):
                return None
        return urlunparse(("", "", parts.path,
            parts.params, parts.query, parts.fragment))