static void simple_free_urb(struct urb *urb)
{
	unsigned long offset = buffer_offset(urb->transfer_buffer);

	if (urb->transfer_flags & URB_NO_TRANSFER_DMA_MAP)
		usb_free_coherent(
			urb->dev,
			urb->transfer_buffer_length + offset,
			urb->transfer_buffer - offset,
			urb->transfer_dma - offset);
	else
		kfree(urb->transfer_buffer - offset);
	usb_free_urb(urb);
}

The product dereferences a pointer that it expects to be valid but is NULL.