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。
/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:24:16: warning: returning 'volatile int *' from a function with result type 'int *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
return &__pico_errno;
^~~~~~~~~~~~~
/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:30:16: warning: returning 'volatile int *' from a function with result type 'int *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
return &__pico_errno;
^~~~~~~~~~~~~
/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:33:12: warning: incompatible pointer types returning 'rt_err_t *' (aka 'long *') from a function with result type 'int *' [-Wincompatible-pointer-types]
return &tid->error;
This option is for RT-Thread Nano version.
If select this option, it will not compile components/libc
folder and only use tool chain internal libc. Normally, the
tool chain internal is only cover ISO standard (e.g. armcc),
but some tool chains' internal libc will cover more than
ISO standard (e.g. newlib).
