gofmt: don't substitute invalid positions with valid ones in rewrites
gofmt: don't substitute invalid positions with valid ones in rewrites

Fixes rewrite bug: 'f(x) -> f(0)' where functions "picked up" ... arguments.

R=rsc
CC=golang-dev
http://codereview.appspot.com/2279041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go