Fix prevent issue

Fix prevent issue
400008 Dereference null return value - flat_navi.c:debug
451475 Dereference null return value - screen_reader_system.c:tapi_init
451483 Dereference null return value - screen_reader_system.c:device_time_get
451485 Dereference null return value - screen_reader_system.c:device_date_get
451637 Structurally dead code - screen_reader_system.c:device_error_to_string

Change-Id: Ic99449307efdf315c84812005742c6b99ed873e9