io/ioutil: fix bug in ReadFile when Open succeeds but Stat fails
io/ioutil: fix bug in ReadFile when Open succeeds but Stat fails

R=gri
CC=golang-dev
http://codereview.appspot.com/867044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go