html/template: fix panic on Clone

Fixes #3281

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5819044

src/pkg/html/template/clone_test.go

@@ -113,3 +113,10 @@ func TestClone(t *testing.T) {
 		t.Errorf("t3: got %q want %q", got, want)
 	}
 }
+
+// This used to crash; http://golang.org/issue/3281
+func TestCloneCrash(t *testing.T) {
+	t1 := New("all")
+	Must(t1.New("t1").Parse(`{{define "foo"}}foo{{end}}`))
+	t1.Clone()
+}

src/pkg/html/template/template.go

@@ -160,9 +160,11 @@ func (t *Template) Clone() (*Template, error) {
 		if src == nil || src.escaped {
 			return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed", t.Name())
 		}
-		x.Tree = &parse.Tree{
-			Name: x.Tree.Name,
-			Root: x.Tree.Root.CopyList(),
+		if x.Tree != nil {
+			x.Tree = &parse.Tree{
+				Name: x.Tree.Name,
+				Root: x.Tree.Root.CopyList(),
+			}
 		}
 		ret.set[name] = &Template{
 			false,