fixup! Fix segfault in State::serialize method

conn.go

@@ -410,6 +410,7 @@ func (c *Conn) Close() error {
 }
 
 // ConnectionState returns basic DTLS details about the connection.
+// If a handshake has not yet been completed, ConnectionState returns nil.
 // Note that this replaced the `Export` function of v1.
 func (c *Conn) ConnectionState() *State {
 	if !c.isHandshakeCompletedSuccessfully() {