Packet with zero data length (espressif#1659)

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.

Author: Schuemi

libraries/WiFi/src/WiFiUdp.cpp

@@ -221,6 +221,7 @@ int WiFiUDP::parsePacket(){
  }
  remote_ip = IPAddress(si_other.sin_addr.s_addr);
  remote_port = ntohs(si_other.sin_port);
+ if (len == 0) return 0;
  rx_buffer = new cbuf(len);
  rx_buffer->write(buf, len);
  delete[] buf;
@@ -264,6 +265,7 @@ int WiFiUDP::peek(){
 }
 
 void WiFiUDP::flush(){
+ if(!rx_buffer) return;
  cbuf *b = rx_buffer;
  rx_buffer = 0;
  delete b;