zeroconf/Bonjour: don't call DNSServiceRefDeallocate() if DNSServiceRegister() fails

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.

src/zeroconf/Bonjour.cxx

@@ -97,11 +97,6 @@ BonjourInit(EventLoop &loop, const char *service_name, unsigned port)
 	if (error != kDNSServiceErr_NoError) {
 		LogError(bonjour_domain,
 			 "Failed to register zeroconf service");
-
-		if (dnsReference) {
-			DNSServiceRefDeallocate(dnsReference);
-			dnsReference = nullptr;
-		}
 		return;
 	}