we do have ValidCol(), so use it

Change-Id: I50d39f7022edcf851f299fc3589e43bf95f682b5
how about style?

Change-Id: I2b53f87f0bf0838c0ef61c3b31408643ae4a0fc3
there is no need to expose an ScColumn*, and always check for validity

Change-Id: Id7f4f180083ec61a98cc93dd7c41b66cbac28953