Simple Item Selector: save selected index even on cancel

This is a better fix for bug #25536 since it allows easy bypassing of the dialog with Esc if single button
mode was enabled, but without crashing. Usecases can stil explicitly check for the dialog retval for conditional
behavior.

src/gui/dialogs/simple_item_selector.cpp

@@ -120,12 +120,8 @@ void simple_item_selector::pre_show(window& window)
 
 void simple_item_selector::post_show(window& window)
 {
-	if(get_retval() != window::OK) {
-		return;
-	}
-
-	listbox& list = find_widget<listbox>(&window, "listbox", false);
-	index_ = list.get_selected_row();
+	index_ = find_widget<listbox>(&window, "listbox", false).get_selected_row();
 }
+
 } // namespace dialogs
 } // namespace gui2