text/template: don't panic when function call evaluates a nil pointer

Catch the error instead and return it to the user. Before this fix,
the template package panicked. Now you get:
        template: bug11:1:14: executing "bug11" at <.PS>: dereference of nil pointer of type *string
Extended example at http://play.golang.org/p/uP6pCW3qKT

Fixes #7333.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/64150043

src/pkg/text/template/exec.go

@@ -594,6 +594,9 @@ func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Valu
 		switch {
 		case value.Kind() == reflect.Ptr && value.Type().Elem().AssignableTo(typ):
 			value = value.Elem()
+			if !value.IsValid() {
+				s.errorf("dereference of nil pointer of type %s", typ)
+			}
 		case reflect.PtrTo(value.Type()).AssignableTo(typ) && value.CanAddr():
 			value = value.Addr()
 		default:

src/pkg/text/template/exec_test.go

@@ -512,6 +512,8 @@ var execTests = []execTest{
 	{"bug9", "{{.cause}}", "neglect", map[string]string{"cause": "neglect"}, true},
 	// Field chain starting with function did not work.
 	{"bug10", "{{mapOfThree.three}}-{{(mapOfThree).three}}", "3-3", 0, true},
+	// Dereferencing nil pointer while evaluating function arguments should not panic. Issue 7333.
+	{"bug11", "{{valueString .PS}}", "", T{}, false},
 }
 
 func zeroArgs() string {
@@ -546,6 +548,11 @@ func vfunc(V, *V) string {
 	return "vfunc"
 }
 
+// valueString takes a string, not a pointer.
+func valueString(v string) string {
+	return "value is ignored"
+}
+
 func add(args ...int) int {
 	sum := 0
 	for _, x := range args {
@@ -580,17 +587,18 @@ func mapOfThree() interface{} {
 func testExecute(execTests []execTest, template *Template, t *testing.T) {
 	b := new(bytes.Buffer)
 	funcs := FuncMap{
-		"add":        add,
-		"count":      count,
-		"dddArg":     dddArg,
-		"echo":       echo,
-		"makemap":    makemap,
-		"mapOfThree": mapOfThree,
-		"oneArg":     oneArg,
-		"stringer":   stringer,
-		"typeOf":     typeOf,
-		"vfunc":      vfunc,
-		"zeroArgs":   zeroArgs,
+		"add":         add,
+		"count":       count,
+		"dddArg":      dddArg,
+		"echo":        echo,
+		"makemap":     makemap,
+		"mapOfThree":  mapOfThree,
+		"oneArg":      oneArg,
+		"stringer":    stringer,
+		"typeOf":      typeOf,
+		"valueString": valueString,
+		"vfunc":       vfunc,
+		"zeroArgs":    zeroArgs,
 	}
 	for _, test := range execTests {
 		var tmpl *Template