io/ioutil: fix crash when Stat fails
io/ioutil: fix crash when Stat fails

Fixes issue 3320.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5824051
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go