[media] vb2: Fix an off by one error in 'vb2_plane_vaddr'

We should ensure that 'plane_no' is '< vb->num_planes' as done in
'vb2_plane_cookie' just a few lines below.

Fixes: e23ccc0a ("[media] v4l: add videobuf2 Video for Linux 2 driver framework")

Cc: stable@vger.kernel.org
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>