Fix case where a master isn't available

dns.go

@@ -243,6 +243,8 @@ func (m *DNSMetrics) Collect() {
 
 	for ix, server := range dns_servers {
 		if server.role == "master" {
+			// check that a result was actually received by the master
+			if results[ix] != nil {
 				soa := results[ix].serial
 				masters = append(masters, soa)
 				if soa > latest_soa {
@@ -250,6 +252,7 @@ func (m *DNSMetrics) Collect() {
 				}
 			}
 		}
+	}
 
 	//	fmt.Printf("latest_soa %d\n", latest_soa)