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

[ComerLater]

拉取/合并请求描述：(PR description)

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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。

/* If not use RT_SERIAL_TX_BLOCKING_BUFFER,
 * the control() API is called to configure the serial device */
tx_fifo = (struct rt_serial_tx_fifo*) rt_malloc
        (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;

请提供验证的bsp和config (provide the config and bsp)

• BSP: bsp/stm32/stm32h743-atk-apollo

• .config:

• action:

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• [ ] 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• [x] 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• [x] 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• [x] 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• [x] 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• [x] 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• [x] 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• [x] 代码是高质量的 Code in this PR is of high quality
• [x] 已经使用formatting 等源码格式化工具确保格式符合RT-Thread代码规范 This PR complies with RT-Thread code specification

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[kurisaW]

请您签署一下github CLA @drjlab

[ComerLater]

请您签署一下github CLA @drjlab

已经完成github CLA签署了，谢谢