text/template: fix nil crash on Templates

Fixes #3872.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6612060

src/pkg/text/template/exec_test.go

@@ -811,3 +811,8 @@ func TestTree(t *testing.T) {
 		t.Errorf("expected %q got %q", expect, result)
 	}
 }
+
+func TestExecuteOnNewTemplate(t *testing.T) {
+	// This is issue 3872.
+	_ = New("Name").Templates()
+}

src/pkg/text/template/template.go

@@ -117,6 +117,9 @@ func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error
 // Templates returns a slice of the templates associated with t, including t
 // itself.
 func (t *Template) Templates() []*Template {
+	if t.common == nil {
+		return nil
+	}
 	// Return a slice so we don't expose the map.
 	m := make([]*Template, 0, len(t.tmpl))
 	for _, v := range t.tmpl {