diskselector: Fix segmentation fault

diskselector: Fix segmentation fault

Summary:
The code was missing null check of sd->items list.
If the deleted item is the only item of the list,
sd->items becomes empty after removing it.
In that case, sd->selected_item should be set as NULL.
@fix
Fixes T988

Test Plan: execute diskselector_example_02 > click "Delete item" button 3 times

Reviewers: raster, seoz

Reviewed By: seoz

CC: seoz
Maniphest Tasks: T988

Differential Revision: https://phab.enlightenment.org/D595