Handle failure to dial.

zfs_replication_exporter.go

@@ -195,10 +195,12 @@ func main() {
 						glog.Errorf("Error dialing %q: %v", host, err)
 					}
 				}
-				if err := updateMetrics(host, c); err != nil {
-					glog.Errorf("Failed to update metrics: %v", err)
-					c.Close()
-					c = nil
+				if c != nil {
+					if err := updateMetrics(host, c); err != nil {
+						glog.Errorf("Failed to update metrics: %v", err)
+						c.Close()
+						c = nil
+					}
 				}
 				time.Sleep(*refreshInterval)
 			}