[release-branch.go1.16] text/template: initialize template before locking it

For #39807
For #48436
Fixes #48443

Change-Id: I75f82fd8738dd2f11f0c69b1230e1be1abc36024
Reviewed-on: https://go-review.googlesource.com/c/go/+/350730
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
(cherry picked from commit ba1c52d7d7)
Reviewed-on: https://go-review.googlesource.com/c/go/+/351116

src/text/template/multi_test.go

@@ -452,3 +452,13 @@ func TestIssue19294(t *testing.T) {
 		}
 	}
 }
+
+// Issue 48436
+func TestAddToZeroTemplate(t *testing.T) {
+	tree, err := parse.Parse("c", cloneText3, "", "", nil, builtins())
+	if err != nil {
+		t.Fatal(err)
+	}
+	var tmpl Template
+	tmpl.AddParseTree("x", tree["c"])
+}

src/text/template/template.go

@@ -127,9 +127,9 @@ func (t *Template) copy(c *common) *Template {
 // its definition. If it has been defined and already has that name, the existing
 // definition is replaced; otherwise a new template is created, defined, and returned.
 func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
+	t.init()
 	t.muTmpl.Lock()
 	defer t.muTmpl.Unlock()
-	t.init()
 	nt := t
 	if name != t.name {
 		nt = t.New(name)