Fix two minor errors in hostname validation.

RFC 6125 says that if the host is an IP address, a subjectAltName of
 type iPAddress must (no 2119 caps) be present and must be used.
We would still fall back to checking the Common Name if no
 subjectAltName was present.

https://marc.info/?l=openssl-dev&m=139617145216047&w=2 interprets
 RFC 6125 to say that if the host is a DNS name, but the certificate
 only contains a subjectAltName of type iPAddress, then we should
 still fall back to checking the Common Name.
We would only check the Common Name if there was no subjectAltName
 of any type.

Restructure the hostname validation to check IP addresses up-front
 and fall back to checking the Common Name in the proper cases.