If you receive a package with a data length of zero, parsePacket returns 0, but rx_buffer will exist. So if another parsePacket with no read access returns to zeros, there is still data that can be read. This example would not work: https://www.arduino.cc/en/Reference/EthernetUDPParsePacket
Also I added a check if rx_buffer exit when you try to flush it.
f1f8d7e306