vb2: Fix an off by one error in 'vb2_plane_vaddr'

commit 5ebb6dd36c9f5fb37b1077b393c254d70a14cb46 upstream.

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

Fixes: e23ccc0ad925 ("[media] v4l: add videobuf2 Video for Linux 2 driver framework")

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>
[bwh: Backported to 3.2: adjust filename, context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>