USB: lvtest: fix NULL-deref at probe

Make sure to check the number of endpoints to avoid dereferencing a
NULL-pointer should the probed device lack endpoints.

Note that this driver does not bind to any devices by default.

Fixes: ce21bfe6 ("USB: Add LVS Test device driver")
Cc: stable <stable@vger.kernel.org>     # 3.17
Cc: Pratyush Anand <pratyush.anand@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>