message: fix assert fault when config.enable_uart0 == false
diff: @@ -21,7 +21,7 @@ static uint32_t dma_last_rcv_idx;
  // FIXME: the stdlib doesn't know about CMSIS threads, so this is just a global variable
  // static thread_local uint32_t deadline_ms = 0;
  -osThreadId uart_thread;
  +osThreadId uart_thread = 0;
  extern UART_HandleTypeDef* uart0;
  static UART_HandleTypeDef* huart_ = uart0; // defined in board.cpp.
  const uint32_t stack_size_uart_thread = 4096; // Bytes
  @@ -115,8 +115,10 @@ void start_uart_server() {
  }
  void uart_poll() {
  + if (uart_thread) { // the thread is only started if UART is enabled
  osThreadResume(uart_thread);
  }
  +}
  void HAL_UART_TxCpltCallback(UART_HandleTypeDef* huart) {
  osSemaphoreRelease(sem_uart_dma);
  