usb: misc: usbtest: fix error of urb allocation

urb allocation will fail when usbtest_alloc_urb() tries to
allocate zero length buffer, but it doesn't need it in fact,
so just skips buffer allocation in the case.
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/misc/usbtest.c

@@ -287,6 +287,9 @@ static struct urb *usbtest_alloc_urb(
 	if (usb_pipein(pipe))
 		urb->transfer_flags |= URB_SHORT_NOT_OK;
 
+	if ((bytes + offset) == 0)
+		return urb;
+
 	if (urb->transfer_flags & URB_NO_TRANSFER_DMA_MAP)
 		urb->transfer_buffer = usb_alloc_coherent(udev, bytes + offset,
 			GFP_KERNEL, &urb->transfer_dma);