helo: avoid undef warning when rDNS is invalid
helo: avoid undef warning when rDNS is invalid

specifically, when rDNS returns an invalid FQDN like 'null.', which doesn't have
a domain part.

sieutruc/qpsmtpd,msimerson/qpsmtpd,salvis/qpsmtpd,priyadi/qpsmtpd,sieutruc/qpsmtpd,priyadi/qpsmtpd,smtpd/qpsmtpd,smtpd/qpsmtpd,priyadi/qpsmtpd,salvis/qpsmtpd,sieutruc/qpsmtpd,msimerson/qpsmtpd