text/template: fix nil error on redefinition

Fixes #2720.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5545072

src/pkg/text/template/multi_test.go

@@ -9,6 +9,7 @@ package template
 import (
 	"bytes"
 	"fmt"
+	"strings"
 	"testing"
 	"text/template/parse"
 )
@@ -257,3 +258,17 @@ func TestAddParseTree(t *testing.T) {
 		t.Errorf("expected %q got %q", "broot", b.String())
 	}
 }
+
+func TestRedefinition(t *testing.T) {
+	var tmpl *Template
+	var err error
+	if tmpl, err = New("tmpl1").Parse(`{{define "test"}}foo{{end}}`); err != nil {
+		t.Fatalf("parse 1: %v", err)
+	}
+	if _, err = tmpl.New("tmpl2").Parse(`{{define "test"}}bar{{end}}`); err == nil {
+		t.Fatal("expected error")
+	}
+	if !strings.Contains(err.Error(), "redefinition") {
+		t.Fatalf("expected redefinition error; got %v", err)
+	}
+}

src/pkg/text/template/template.go

@@ -198,7 +198,7 @@ func (t *Template) associate(new *Template) error {
 	name := new.name
 	if old := t.tmpl[name]; old != nil {
 		oldIsEmpty := parse.IsEmptyTree(old.Root)
-		newIsEmpty := parse.IsEmptyTree(new.Root)
+		newIsEmpty := new.Tree != nil && parse.IsEmptyTree(new.Root)
 		if !oldIsEmpty && !newIsEmpty {
 			return fmt.Errorf("template: redefinition of template %q", name)
 		}