negative timestamps on messages fix

* Negative timestamps on messages are invalid thus this commit adds
  checks for them. On decoding, this returns a zero time for
  negative timestamps for easy validity checking. On encoding, zero
  times will be encoded as -1, anything else before Unix epoch
  returns an error.

message.go

@@ -45,7 +45,15 @@ func (m *Message) encode(pe packetEncoder) error {
 	pe.putInt8(attributes)
 
 	if m.Version >= 1 {
-		pe.putInt64(m.Timestamp.UnixNano() / int64(time.Millisecond))
+		timestamp := int64(-1)
+
+		if !m.Timestamp.Before(time.Unix(0, 0)) {
+			timestamp = m.Timestamp.UnixNano() / int64(time.Millisecond)
+		} else if !m.Timestamp.IsZero() {
+			return PacketEncodingError{fmt.Sprintf("invalid timestamp (%v)", m.Timestamp)}
+		}
+
+		pe.putInt64(timestamp)
 	}
 
 	err := pe.putBytes(m.Key)
@@ -125,7 +133,15 @@ func (m *Message) decode(pd packetDecoder) (err error) {
 		if err != nil {
 			return err
 		}
-		m.Timestamp = time.Unix(millis/1000, (millis%1000)*int64(time.Millisecond))
+
+		// negative timestamps are invalid, in these cases we should return
+		// a zero time
+		timestamp := time.Time{}
+		if millis >= 0 {
+			timestamp = time.Unix(millis/1000, (millis%1000)*int64(time.Millisecond))
+		}
+
+		m.Timestamp = timestamp
 	}
 
 	m.Key, err = pd.getBytes()