udev_enumerate_new() does not check its input argument of a 'struct udev' for NULLness.

This means that arguably sloppy code like in Xorg:

    udev = udev_monitor_get_udev(udev_monitor);
    enumerate = udev_enumerate_new(udev);
    if (!enumerate)
        return;

Will have a valid (but unusable) enumerate object even if the 'udev' pointer itself is NULL.

While the Xorg code should probably also check the 'udev' pointer, I think it makes sense for libudev to not create/return a bogus enumerate object.  Plus, the other functions in libudev are pretty good about validating their input, so it would be consistent.

Patch coming.