text/template: don't panic when using AddParseTree with an unparsed template

Fixes #7032.

R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/43960045

src/pkg/text/template/multi_test.go

@@ -259,6 +259,18 @@ func TestAddParseTree(t *testing.T) {
 	}
 }
 
+// Issue 7032
+func TestAddParseTreeToUnparsedTemplate(t *testing.T) {
+	master := "{{define \"master\"}}{{end}}"
+	tmpl := New("master")
+	tree, err := parse.Parse("master", master, "", "", nil)
+	if err != nil {
+		t.Fatalf("unexpected parse err: %v", err)
+	}
+	masterTree := tree["master"]
+	tmpl.AddParseTree("master", masterTree) // used to panic
+}
+
 func TestRedefinition(t *testing.T) {
 	var tmpl *Template
 	var err error

src/pkg/text/template/template.go

@@ -105,7 +105,7 @@ func (t *Template) copy(c *common) *Template {
 // AddParseTree creates a new template with the name and parse tree
 // and associates it with t.
 func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
-	if t.tmpl[name] != nil {
+	if t.common != nil && t.tmpl[name] != nil {
 		return nil, fmt.Errorf("template: redefinition of template %q", name)
 	}
 	nt := t.New(name)