text/template: fix bug in pipelined variadics

Simple bug in argument processing: The final arg may
be the pipeline value, in which case it gets bound to the
fixed argument section. The code got that wrong. Easy
to fix.

Fixes #8950.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/161750043

src/text/template/exec.go

@@ -546,7 +546,7 @@ func (s *state) evalCall(dot, fun reflect.Value, node parse.Node, name string, a
 	argv := make([]reflect.Value, numIn)
 	// Args must be evaluated. Fixed args first.
 	i := 0
-	for ; i < numFixed; i++ {
+	for ; i < numFixed && i < len(args); i++ {
 		argv[i] = s.evalArg(dot, typ.In(i), args[i])
 	}
 	// Now the ... args.

src/text/template/exec_test.go

@@ -893,6 +893,18 @@ func TestMessageForExecuteEmpty(t *testing.T) {
 	}
 }
 
+func TestFinalForPrintf(t *testing.T) {
+	tmpl, err := New("").Parse(`{{"x" | printf}}`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var b bytes.Buffer
+	err = tmpl.Execute(&b, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
 type cmpTest struct {
 	expr  string
 	truth string