media: v4l: subdev: Fail graciously when getting try data for NULL state

The state argument for the functions for obtaining various parts of the
state is NULL if it is called by drivers for active state. Fail graciously
in that case instead of dereferencing a NULL pointer.

Suggested-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>