elm_focus_legacy: check if top is really a window

Summary:
if not, things are going to fall apart, as manager_top then can be NULL
or invalid. Top has to be a window element, if this is not the case,
then the widget tree of the given widget is dangling somewhere in the
void. Calculating the next object in there or even the active manager
will result in errors, since the active manager is not really the active
manager, but rather just a manager object somewhere in a danging widget
subtree.

Moving the focus into such a dangling widgettree might result in a stuck
focus rect on this object, since the DFS of the focus manager
implementation cannot backtrack anymore into the widgets that are still
part of the widget graph.

Depends on D6531

Reviewers: devilhorns, segfaultxavi, zmike

Reviewed By: segfaultxavi

Subscribers: cedric, #committers, zmike

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D6532

src/lib/elementary/elm_focus_legacy.c

@@ -184,6 +184,8 @@ elm_object_focus_next(Evas_Object        *obj,
    API_ENTRY()
 
    top = elm_object_top_widget_get(obj);
+   EINA_SAFETY_ON_FALSE_RETURN(efl_isa(top, EFL_UI_WIN_CLASS));
+
    manager_top = efl_ui_focus_util_active_manager(EFL_UI_FOCUS_UTIL_CLASS, obj);
    logical = efl_ui_focus_manager_focus_get(manager_top);