Fix switch_split_user_domain handling of sips: URLs

In commit 7efeabbd88e81ee368de6ced32fed06c8035097b Anthony fixed the
handling of sip:example.com and sips:example.com URLs, however he
introduced a regression causing URLs starting with 's' to be parsed
incorrectly.

In commit 7d2456ea27c092825c8d614ac6eee71547374464 Brian fixed the
regression, but introduced a regression causing sips:example.com URLs
to be handled incorrectly.