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>