[Fix prevent issues]

[Fix prevent issues]

of navigator.c
 - CID 389096 Dereference null return value
 - CID 397278 Dereference null return value
 - CID 399629 Dereference null return value
 - CID 436264 Dereference null return value
 - CID 436266 Dereference null return value
 - CID 436267 Dereference null return value
 - CID 436277 Dereference null return value
 - CID 436278 Dereference null return value
 - CID 436281 Dereference null return value
 - CID 436282 Dereference null return value
 - CID 436404 Resource leak
 * You don't need to check NULL when you use g_object_unref. Because g_return_if_fail (G_IS_OBJECT (object)) prevents the dereference NULL.

of object_cache.c
 - CID 436449 Dereference before null check

of position_sort.c
 - CID 384795 Dereference null return value
 - CID 392419 Dereference null return value
 - CID 402450 Dereference null return value

of screen_reader_spi.c
 - CID 387658 Dereference null return value
 - CID 389289 Dereference null return value
 - CID 396599 Dereference null return value
 - CID 397835 Dereference null return value
 - CID 400614 Dereference null return value
 - CID 404016 Uninitialized pointer read
 - CID 435865 Resource leak

of screen_reader_tts.c
 - CID 387027 Dereference null return value

Change-Id: I8b6b82d770fda31ddc2615539e26d4529353db78