WiFiClient - Properly initialize and check _rxBuffer

[lucasssvaz]

Description of Change

Fixes LoadProhibited exception (introduced in #8541) by properly initializing and checking the _rxBuffer pointer in the WiFiClient.

Tests scenarios

Tested on ESP32-DevKitC using the following sketch:

#include <Arduino.h>
#include <WiFi.h>

WiFiClient client;

void loop() {
  delay(1000);
  Serial.println(".");
}

void setup() {
  Serial.begin(115200);
  Serial.println("READY");
  delay(1000);
  Serial.println("Flush WiFiClient client...");
  client.flush();
  delay(2000);
  Serial.println("Setup complete");
}

Related links

Closes #8681