Fix WiFiClient error handling (espressif#1388)

Author: chemicstry

libraries/WiFi/src/WiFiClient.cpp

@@ -313,7 +313,7 @@ uint8_t WiFiClient::connected()
  if (_connected) {
  uint8_t dummy;
  int res = recv(fd(), &dummy, 0, MSG_DONTWAIT);
- if (res <= 0) {
+ if (res < 0) {
  switch (errno) {
  case ENOTCONN:
  case EPIPE:
@@ -328,7 +328,6 @@ uint8_t WiFiClient::connected()
  }
  }
  else {
- // Should never happen since requested 0 bytes
  _connected = true;
  }
  }