*** Submitted as
http://code.google.com/p/go/source/detail?r=616adac0bb59 ***
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
http://codereview.appspot.com/6855099/
