media: v4l: ioctl: Validate num_planes before using it

The for loop to reset the memory of the plane reserved fields runs over
num_planes provided by the user without validating it. Ensure num_planes
is no more than VIDEO_MAX_PLANES before the loop.

Fixes: 4e1e0eb0

 ("media: v4l2-ioctl: Zero v4l2_plane_pix_format reserved fields")

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>