WiFiClient - Properly initialize and check _rxBuffer (#8699)

libraries/WiFi/src/WiFiClient.cpp

@@ -182,7 +182,7 @@ public:
     }
 };
 
-WiFiClient::WiFiClient():_connected(false),_timeout(WIFI_CLIENT_DEF_CONN_TIMEOUT_MS),next(NULL)
+WiFiClient::WiFiClient():_rxBuffer(nullptr),_connected(false),_timeout(WIFI_CLIENT_DEF_CONN_TIMEOUT_MS),next(NULL)
 {
 }
 
@@ -508,7 +508,9 @@ int WiFiClient::available()
 // Though flushing means to send all pending data,
 // seems that in Arduino it also means to clear RX
 void WiFiClient::flush() {
+    if (_rxBuffer != nullptr) {
         _rxBuffer->flush();
+    }
 }
 
 uint8_t WiFiClient::connected()