fix(serial_v2): tx_fifo->rb.buffer_ptr not init when RT_DEVICE_FLAG_DMA_TX and RT_SERIAL_TX_BLOCKING

为什么提交这份PR (why to submit this PR)
serial_v2驱动框架，rt_serial_tx_enable()函数，在 DMA阻塞发送(RT_DEVICE_FLAG_DMA_TX and RT_SERIAL_TX_BLOCKING) 的时候，使用rt_malloc分配了tx_fifo内存，但是并未对tx_fifo->rb.buffer_ptr指针进行初始化（rt_malloc不会自动初始化内存），因此可能导致buffer_ptr是一个随机指针。但是rt_serail_write()函数需要根据tx_fifo->rb.buffer_ptr是否为RT_NULL来判断是调用_serial_fifo_tx_blocking_nbuf()还是_serial_fifo_tx_blocking_buf()。

由于tx_fifo->rb.buffer_ptr可能是一个随机值（不一定为RT_NULL=0），导致RT_DEVICE_FLAG_DMA_TX and RT_SERIAL_TX_BLOCKING模式下调用了_serial_fifo_tx_blocking_buf()，但是rb没有真正开辟内存空间，导致内存异常

你的解决方案是什么 (what is your solution)
rt_serial_tx_enable()，在DMA阻塞发送分支下，对tx_fifo->rb.buffer_ptr进行初始化，赋值为RT_NULL。

components/drivers/serial/serial_v2.c

@@ -684,6 +684,9 @@ static rt_err_t rt_serial_tx_enable(struct rt_device        *dev,
                     (sizeof(struct rt_serial_tx_fifo));
             RT_ASSERT(tx_fifo != RT_NULL);
 
+            /* Init rb.buffer_ptr to RT_NULL, in rt_serial_write() need check it
+             * otherwise buffer_ptr maybe a random value, as rt_malloc not init memory */
+            tx_fifo->rb.buffer_ptr = RT_NULL;
             serial->serial_tx = tx_fifo;
 
 #ifndef RT_USING_DEVICE_OPS

components/drivers/usb/usbdevice/class/cdc_vcom.c

@@ -755,7 +755,11 @@ static rt_ssize_t _vcom_rb_block_put(struct vcom *data, const rt_uint8_t *buf, r
     return size;
 }
 
-static rt_ssize_t _vcom_tx(struct rt_serial_device *serial, rt_uint8_t *buf, rt_size_t size, int direction)
+#ifdef RT_USING_SERIAL_V1
+static rt_size_t _vcom_tx(struct rt_serial_device *serial, rt_uint8_t *buf, rt_size_t size, int direction)
+#else
+static rt_size_t _vcom_tx(struct rt_serial_device *serial, rt_uint8_t *buf, rt_size_t size, rt_uint32_t tx_flag)
+#endif
 {
     struct ufunction *func;
     struct vcom *data;