fix(lwip): Add early out in NetworkUDP::parsePacket() when socket h…

…as no data (espressif#10075)

* 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.

* fix(lwip): Initialize `len` to ensure it's set before check

libraries/Network/src/NetworkUdp.cpp

@@ -296,7 +296,14 @@ int NetworkUDP::parsePacket() {
   }
   struct sockaddr_storage si_other_storage;  // enough storage for v4 and v6
   socklen_t slen = sizeof(sockaddr_storage);
-  int len;
+  int len = 0;
+  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;