[media] vb2: only check ret if we assigned it

Move the ret check to the right level under if (pb). It is not
used by the code before that point if pb is NULL.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>