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

src/lib/elementary/elm_genlist.c

@@ -7691,7 +7691,8 @@ _filter_item_internal(Elm_Gen_Item *it)
                 WIDGET(it), sd->filter_data)))
           {
              it->hide = EINA_TRUE;
-             it->item->block->changed = EINA_TRUE;
+             if (it->item->block)
+               it->item->block->changed = EINA_TRUE;
           }
         else
           sd->filtered_count++;
@@ -7722,7 +7723,8 @@ _item_filtered_get(Elm_Gen_Item *it)
           }
 
         _filter_item_internal(it);
-        it->item->block->changed = EINA_TRUE;
+        if (it->item->block)
+          it->item->block->changed = EINA_TRUE;
         ELM_SAFE_FREE(sd->calc_job, ecore_job_del);
         sd->calc_job = ecore_job_add(_calc_job, sd->obj);
    }