[media] cx231xx: correctly handling failed allocation

Since kmalloc can be failed in memory pressure,
if not properly handled, NULL dereference can be happend

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