go/ast: minor cleanup
go/ast: minor cleanup

It's better to use IsValid() then checking a (possibly
partially set up) position against NoPos directly.

R=dsymonds
CC=golang-dev
http://codereview.appspot.com/6855099
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go