media: dvb_vb2: fix possible out of bound access

commit 37238699 upstream.

vb2_core_qbuf and vb2_core_querybuf don't check the range of b->index
controlled by the user.

Fix this by adding range checking code before using them.

Fixes: 57868acc ("media: videobuf2: Add new uAPI for DVB streaming I/O")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>