dont't panic when marshal from nil or unmarshal to nil interface or pointer (#353)

marshal.go

@@ -281,6 +281,9 @@ func (e *Encoder) SetTagMultiline(v string) *Encoder {
 
 func (e *Encoder) marshal(v interface{}) ([]byte, error) {
 	mtype := reflect.TypeOf(v)
+	if mtype == nil {
+		return []byte{}, errors.New("nil cannot be marshaled to TOML")
+	}
 
 	switch mtype.Kind() {
 	case reflect.Struct, reflect.Map:
@@ -288,6 +291,9 @@ func (e *Encoder) marshal(v interface{}) ([]byte, error) {
 		if mtype.Elem().Kind() != reflect.Struct {
 			return []byte{}, errors.New("Only pointer to struct can be marshaled to TOML")
 		}
+		if reflect.ValueOf(v).IsNil() {
+			return []byte{}, errors.New("nil pointer cannot be marshaled to TOML")
+		}
 	default:
 		return []byte{}, errors.New("Only a struct or map can be marshaled to TOML")
 	}
@@ -538,6 +544,9 @@ func (d *Decoder) SetTagName(v string) *Decoder {
 
 func (d *Decoder) unmarshal(v interface{}) error {
 	mtype := reflect.TypeOf(v)
+	if mtype == nil {
+		return errors.New("nil cannot be unmarshaled from TOML")
+	}
 	if mtype.Kind() != reflect.Ptr {
 		return errors.New("only a pointer to struct or map can be unmarshaled from TOML")
 	}
@@ -550,6 +559,10 @@ func (d *Decoder) unmarshal(v interface{}) error {
 		return errors.New("only a pointer to struct or map can be unmarshaled from TOML")
 	}
 
+	if reflect.ValueOf(v).IsNil() {
+		return errors.New("nil pointer cannot be unmarshaled from TOML")
+	}
+
 	vv := reflect.ValueOf(v).Elem()
 
 	sval, err := d.valueFromTree(elem, d.tval, &vv)

marshal_test.go

@@ -2772,3 +2772,21 @@ InnerField2 = 2048
 		t.Errorf("Bad unmarshal: expected %v, got %v", expected, actual)
 	}
 }
+
+func TestMarshalNil(t *testing.T) {
+	if _, err := Marshal(nil); err == nil {
+		t.Errorf("Expected err from nil marshal")
+	}
+	if _, err := Marshal((*struct{})(nil)); err == nil {
+		t.Errorf("Expected err from nil marshal")
+	}
+}
+
+func TestUnmarshalNil(t *testing.T) {
+	if err := Unmarshal([]byte(`whatever = "whatever"`), nil); err == nil {
+		t.Errorf("Expected err from nil marshal")
+	}
+	if err := Unmarshal([]byte(`whatever = "whatever"`), (*struct{})(nil)); err == nil {
+		t.Errorf("Expected err from nil marshal")
+	}
+}