Update WiFiClient.cpp (#3608)

fixed the connected() function so that it only checks errno if recv returns a value of -1.

"in the even of an error, errno is set to indicate the error" --manpage

This fixes the ESP32 Webserver when dealing with a modern webserver with a slow SD card.

libraries/WiFi/src/WiFiClient.cpp

@@ -495,6 +495,8 @@ uint8_t WiFiClient::connected()
         int res = recv(fd(), &dummy, 0, MSG_DONTWAIT);
         // avoid unused var warning by gcc
         (void)res;
+        // recv only sets errno if res is -1
+        if (res < 0){
           switch (errno) {
               case EWOULDBLOCK:
               case ENOENT: //caused by vfs
@@ -513,6 +515,9 @@ uint8_t WiFiClient::connected()
                   _connected = true;
                   break;
           }
+        } else {
+          _connected = true;
+        }
     }
     return _connected;
 }