Log Message:
Avoid re-closing a udev that already was closed.  Avoids a double-close
that triggers a SEGV in a list-delete operation.

This may actually be the real bug for this code in libusb1.c:

static int nut_libusb_open(libusb_device_handle **udevp,
[ ... ]
#ifndef __linux__ /* SUN_LIBUSB (confirmed to work on Solaris and FreeBSD) */
       /* Causes a double free corruption in linux if device is detached! */

as the same double-free list issue occurs here.