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

apparentlymart/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,tv42/old-go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go