media: vpbe_venc: potential uninitialized variable in ven_sub_dev_init()

Smatch complains that "venc" could be uninitialized.  There are a couple
of error paths where it looks like maybe that could happen.  I don't know
if it's really a bug, but it's reasonable to set "venc" to NULL and
silence the warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>