NativeSerialPort: synchronize close()

src/org/hwo/io/NativeSerialPort.java

@@ -80,7 +80,7 @@ public class NativeSerialPort extends SerialPort {
 	}
 
 	@Override
-	public void close() {
+	public synchronized void close() {
 		if (nativeHandle >= 0)
 		{
 			System.err.println(String.format("NativeSerialPort: Closing nativeHandle %d",nativeHandle));