Change minimal packet size to 4 bytes

A valid packet without token+options+payload will takes only 4 bytes.

The bug was found with the help of go-fuzz
(https://github.com/dvyukov/go-fuzz).

message.go

@@ -553,7 +553,7 @@ func parseMessage(data []byte) (Message, error) {
 
 // UnmarshalBinary parses the given binary slice as a Message.
 func (m *Message) UnmarshalBinary(data []byte) error {
-	if len(data) < 6 {
+	if len(data) < 4 {
 		return errors.New("short packet")
 	}
 

message_test.go

@@ -118,6 +118,25 @@ func TestCodeString(t *testing.T) {
 	}
 }
 
+func TestEncodeMessageWithoutOptionsAndPayload(t *testing.T) {
+	req := Message{
+		Type:      Confirmable,
+		Code:      GET,
+		MessageID: 12345,
+	}
+
+	data, err := req.MarshalBinary()
+	if err != nil {
+		t.Fatalf("Error encoding request: %v", err)
+	}
+
+	// Inspected by hand.
+	exp := []byte{0x40, 0x1, 0x30, 0x39}
+	if !bytes.Equal(exp, data) {
+		t.Fatalf("Expected\n%#v\ngot\n%#v", exp, data)
+	}
+}
+
 func TestEncodeMessageSmall(t *testing.T) {
 	req := Message{
 		Type:      Confirmable,
@@ -221,6 +240,30 @@ func TestOptionsWithIllegalLengthAreIgnoredDuringParsing(t *testing.T) {
 	}
 }
 
+func TestDecodeMessageWithoutOptionsAndPayload(t *testing.T) {
+	input := []byte{0x40, 0x1, 0x30, 0x39}
+	msg, err := parseMessage(input)
+	if err != nil {
+		t.Fatalf("Error parsing message: %v", err)
+	}
+
+	if msg.Type != Confirmable {
+		t.Errorf("Expected message type confirmable, got %v", msg.Type)
+	}
+	if msg.Code != GET {
+		t.Errorf("Expected message code GET, got %v", msg.Code)
+	}
+	if msg.MessageID != 12345 {
+		t.Errorf("Expected message ID 12345, got %v", msg.MessageID)
+	}
+	if len(msg.Token) != 0 {
+		t.Errorf("Incorrect token: %q", msg.Token)
+	}
+	if len(msg.Payload) != 0 {
+		t.Errorf("Incorrect payload: %q", msg.Payload)
+	}
+}
+
 func TestDecodeMessageSmallWithPayload(t *testing.T) {
 	input := []byte{
 		0x40, 0x1, 0x30, 0x39, 0x21, 0x3,