Similar to boost::intrusive::safe_link.

This will allow using Zeroconf in output plugins (preparing for
Snapcast with Zeroconf).

According to
https://developer.apple.com/documentation/dnssd/1804733-dnsserviceregister
the DNSServiceRef is initialized only if DNSServiceRegister() returns
kDNSServiceErr_NoError.  The faulty error handling code could
therefore crash.