media: exynos4-is: Prevent NULL pointer dereference in __isp_video_try_fmt()

This patch fixes potential NULL pointer dereference as indicated
by the following static checker warning:

drivers/media/platform/exynos4-is/fimc-isp-video.c:408 isp_video_try_fmt_mplane()
error: NULL dereference inside function '__isp_video_try_fmt(isp, &f->fmt.pix_mp, (0))()'.

Fixes: 34947b8a: ("[media] exynos4-is: Add the FIMC-IS ISP capture DMA driver")

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>