fix(lwip): Add early out in NetworkUDP::parsePacket() when socket has no data.

Previously, `NetworkUDP::parsePacket()` would take the time to allocate a 1460 byte buffer
to call `recvfrom()` with, immediately freeing it if there was no data read.

This change has it check if there is available data via `ioctl()` with `FIONREAD` first,
saving the allocation and thus significantly increasing performance in no data situations.

Author: nitz

libraries/Network/src/NetworkUdp.cpp

@@ -297,6 +297,13 @@ int NetworkUDP::parsePacket() {
   struct sockaddr_storage si_other_storage;  // enough storage for v4 and v6
   socklen_t slen = sizeof(sockaddr_storage);
   int len;
+  if (ioctl(udp_server, FIONREAD, &len) == -1) {
+    log_e("could not check for data in buffer length: %d", errno);
+    return 0;
+  }
+  if (!len) {
+    return 0;
+  }
   char *buf = (char *)malloc(1460);
   if (!buf) {
     return 0;