json: fix test if rand returns 0.
json: fix test if rand returns 0.

Fixes test when run with gccgo using optimization, which
changes the order of the calls to rand.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/4639101

wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old