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.
Currently there is bug in WiFiUDP library when you want to use beginPacket(...) without listening on socket (without calling begin(...) first). You can't send any data because socket is not open and also tx_buffer is not allocated which cause crash while writing data to tx_buffer.
