media: rcar-csi2: Add checking to rcsi2_start_receiver()

If rcsi2_code_to_fmt() return NULL, then null pointer dereference occurs
in the next cycle. That should not be possible now but adding checking
protects from future bugs.
The patch adds checking if format is NULL.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Nadezda Lutovinova <lutovinova@ispras.ru>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>