[Fix prevent issue]

[Fix prevent issue]
[CID 451131] flat_navi.c:_contains - dereference null return value
[CID 451136] navigator.c:_focus_next_visible - dereference null return value
[CID 451143] navigator.c:_focus_prev_visible - dereference null return value
[CID 451109] screen_reader_vconf.c:set_language - Array compared against 0
[CID 451144] smart_notification.c:get_realized_items_count - dereference null return value
[CID 451141] window_tracker.c:_get_window_object_from_given - dereference null return value

Change-Id: I64d343a6d163bd8747323b402f6a86141daa9ad5