[media] s5k6aa: Fix possible NULL pointer dereference

Make sure __s5k6aa_get_crop_rect() always returns valid pointer,
as it is assumed at the callers.
crop->which is already validated when subdev set_crop and get_crop
callbacks are called from within the v4l2-core. If it ever happens
the crop operations are called directly for some reason in kernel
space, with incorrect crop->which argument, just log it with WARN
and return reference to the TRY crop.

Reported-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>