[media] vivid: fix buffer overrun

The random_line buffer must be twice the maximum width, but it only allocated
the maximum width, so it was only half the size it needed to be.

Surprisingly I never saw the kernel fail on this, but the same TPG code used in
qv4l2 crashed and valgrind helped me track this bug down.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>