fix(cdc): stop arming OUT endpoint with a NULL buffer

USBD_CDC_Receive() falls back to USBD_CDC_ClearBuffer() when the
receive queue has no room left for another 64 byte block. That helper
re-arms the OUT endpoint with a NULL application buffer:

    USBD_LL_PrepareReceive(pdev, CDC_OUT_EP, 0, 0);

The endpoint is valid again while ep->xfer_buff is NULL. The next OUT
packet the host sends reaches HAL_PCD_IRQHandler(), which copies the
packet without checking the destination:

    if (count != 0U)
    {
      USB_ReadPMA(hpcd->Instance, ep->xfer_buff, ep->pmaadress, count);
    }

The packet lands at address 0 and the write faults. On a STM32F103C8
this is an imprecise bus fault escalated to a hard fault (CFSR
IMPRECISERR, HFSR FORCED), and the board stops answering until reset.

Do not arm the endpoint when there is no room. An unarmed bulk OUT
endpoint NAKs, the host retries, and USBSerial::read() and the
readBytes() family already call CDC_resume_receive() after dequeuing,
so the endpoint is armed with a real block as soon as the sketch
drains. That is the flow control this transfer needs.

Fixes #1399

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: dldntjr407

libraries/USBDevice/src/cdc/usbd_cdc_if.c

@@ -240,10 +240,15 @@ static int8_t USBD_CDC_Receive(uint8_t *Buf, uint32_t *Len)
   /* It always contains required amount of free space for writing */
   CDC_ReceiveQueue_CommitBlock(&ReceiveQueue, (uint16_t)(*Len));
   receivePended = false;
-  /* If enough space in the queue for a full buffer then continue receive */
-  if (!CDC_resume_receive()) {
-    USBD_CDC_ClearBuffer(&hUSBD_Device_CDC);
-  }
+  /*
+   * If there is enough space in the queue for a full packet, continue receive.
+   * Else leave the OUT endpoint unarmed: it then NAKs and the host retries.
+   * USBSerial::read() and the readBytes() family call CDC_resume_receive()
+   * after dequeuing, so the endpoint is armed again with a valid block as soon
+   * as the sketch drains data. Arming it with a NULL buffer instead makes the
+   * low level driver copy the retried packet to address 0.
+   */
+  (void)CDC_resume_receive();
   return ((int8_t)USBD_OK);
 }