efl ui dnd - further fixes on drop list data key handling

the efl ui dnd code didn't reset the drop list key to null (deleting
it) once it had freed the list of frop items. this also fixes another
issue in T7113 which causes crashes on exit.

src/lib/elementary/efl_ui_dnd.c

@@ -401,6 +401,7 @@ _drop_obj_del_cb(void *data EINA_UNUSED, Evas *e EINA_UNUSED, Evas_Object *obj,
         free(drop->pos);
         free(drop);
      }
+   efl_key_data_set(obj, "__drop_list", NULL);
 }
 
 EAPI Eina_Bool