elm_genlist: fix rare segfault

elm_genlist: fix rare segfault

Conditions:
- style is "double_label"
- the is some content for items (i.e elm_label)
- elm_genlist_filter set was once called after genlist creation with
  NULL data
- label_get callback uses elm_genlist_item_prev_get on its current item
- at least one item is added as a sub-item
- ~2 blocks of items are added afterwards
- items are added quickly while holding 'enter' on an elm_button

@fix