bcm2835: camera: check for scene not being found

static analysis by cppcheck detected some potential NULL pointer
dereference issues:

[drivers/media/platform/bcm2835/controls.c:854]: (error) Possible null
  pointer dereference: scene
  (and lines 858, 859 too)

it is possible that scene is not found because of an invalue ctrl->val
and is therefore NULL and hence causing a null pointer dereference.

Signed-off-by: Colin Ian King <colin.king@canonical.com>