efl_ui/button: check icon swallow existence before getting its content

Summary:
some button styles do not have this part

@fix
Depends on D8970

Reviewers: devilhorns

Reviewed By: devilhorns

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D8971

src/lib/elementary/efl_ui_button.c

@@ -416,6 +416,7 @@ _icon_signal_emit(Evas_Object *obj)
    char buf[64];
 
    if (!elm_widget_resize_object_get(obj)) return;
+   if (!edje_object_part_exists(obj, "elm.swallow.content")) return;
    snprintf(buf, sizeof(buf), "elm,state,icon,%s",
             elm_layout_content_get(obj, "icon") ? "visible" : "hidden");