tls: Re-enable check of CN-ID in cert verification

RFC 6125 explicitly states that a client "MUST NOT seek a match
for a reference identifier of CN-ID if the presented identifiers
include a DNS-ID, SRV-ID, URI-ID, or any application-specific
identifier types supported by the client", but it MAY do so if
none of the mentioned identifier types (but others) are present.
crypto: zero is not an error if writing 0 bytes

fix #5128