break from never ending loop. fix #76.

exit from sync_transfer_wait_for_completion when libusb_handle_events_completed failed
and libusb_cancel_transfer returns LIBUSB_ERROR_NOT_FOUND. this error is returned when
urbs is not found in kernel (it was already dequeued from async_completed list),
therefore there is nothing to cancel. if we don't break here libusbx will get stuck in
this loop forever. fix #76.