Set uart queue to NULL after free

Fixes: https://github.com/espressif/arduino-esp32/issues/435

cores/esp32/esp32-hal-uart.c

@@ -226,6 +226,7 @@ void uartEnd(uart_t* uart)
         uint8_t c;
         while(xQueueReceive(uart->queue, &c, 0));
         vQueueDelete(uart->queue);
+        uart->queue = NULL;
     }
 
     uart->dev->conf0.val = 0;