Avoid storing pointer to nil

Before this change, the new test fails with:

	--- FAIL: TestSetOutput (0.00s)
		command_test.go:198: expected setting output to nil to revert back to stdout, got <nil>

command.go

@@ -113,7 +113,7 @@ type Command struct {
 	flagErrorBuf *bytes.Buffer
 
 	args          []string             // actual args parsed from flags
-	output        *io.Writer           // out writer if set in SetOutput(w)
+	output        io.Writer            // out writer if set in SetOutput(w)
 	usageFunc     func(*Command) error // Usage can be defined by application
 	usageTemplate string               // Can be defined by Application
 	flagErrorFunc func(*Command, error) error
@@ -141,7 +141,7 @@ func (c *Command) SetArgs(a []string) {
 // SetOutput sets the destination for usage and error messages.
 // If output is nil, os.Stderr is used.
 func (c *Command) SetOutput(output io.Writer) {
-	c.output = &output
+	c.output = output
 }
 
 // SetUsageFunc sets usage function. Usage can be defined by application.
@@ -199,7 +199,7 @@ func (c *Command) OutOrStderr() io.Writer {
 
 func (c *Command) getOut(def io.Writer) io.Writer {
 	if c.output != nil {
-		return *c.output
+		return c.output
 	}
 	if c.HasParent() {
 		return c.parent.getOut(def)

command_test.go

@@ -191,6 +191,14 @@ func TestEnableCommandSortingIsDisabled(t *testing.T) {
 	EnableCommandSorting = true
 }
 
+func TestSetOutput(t *testing.T) {
+	cmd := &Command{}
+	cmd.SetOutput(nil)
+	if out := cmd.OutOrStdout(); out != os.Stdout {
+		t.Fatalf("expected setting output to nil to revert back to stdout, got %v", out)
+	}
+}
+
 func TestFlagErrorFunc(t *testing.T) {
 
 	cmd := &Command{