io/ioutil: fix data race on rand Fixes issue 4212.
io/ioutil: fix data race on rand
Fixes issue 4212.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6641050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go