[media] s5p-fimc: Avoid null pointer dereference in fimc_capture_ctrls_create()

With presence of some faults, e.g. caused by wrong platform data or
the device tree structure the IDX_SENSOR entry in the array may be NULL,
so make sure it is not dereferenced then.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>