libinput: allow evdev_device_set_output(dev, NULL)

Removing the output association from an evdev_device is more than just
setting the pointer to NULL, one also needs to remove the destroy
listener and flag the destroy listener as unused (notify == NULL).

evdev_device_set_output() can already remove associations, so let it
also handle an assignment to NULL output.

Fix notify_output_destroy() to handle removing an association correctly.
Previously, the listener was left "used", which would mean the next call
to evdev_device_set_output() would have wl_list_remove()'d, accessing
freed memory. This could be triggered by having a touchscreen with a
specified output association, and unplugging then re-plugging the
corresponding output.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>