[media] usbtv: correctly handling failed allocation

Since kzalloc can be failed,
if not properly handled, NULL dereference could be happened.

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>