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: NChunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>