Guard against nil etcd response header (#4339)

Author: amdprophet

cli/commands/cluster/health.go

@@ -1,6 +1,7 @@
 package cluster
 
 import (
+	"errors"
 	"fmt"
 	"io"
 
@@ -23,6 +24,9 @@ func HealthCommand(cli *cli.SensuCli) *cobra.Command {
 			if err != nil {
 				return err
 			}
+			if result.Header == nil {
+				return errors.New("result header was empty, etcd cluster may be down")
+			}
 			err = helpers.PrintTitle(helpers.GetChangedStringValueFlag("format", cmd.Flags()), cli.Config.Format(), fmt.Sprintf("Etcd Cluster ID: %x", result.Header.ClusterId), cmd.OutOrStdout())
 			if err != nil {
 				return err