Implemented getAttachedShaders method.

Also fixed some of the potential null pointer issues if using shader methods on deleted shader objects.

Change-Id: Ied254c1f96f283ba2f48f5a453c38bb7d7981a60
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>