5g: correct size of reg array.
5g: correct size of reg array.

Found by gcc 4.5.2 -Werror build reported on IRC by niemeyer.

R=ken2, rsc, r2
CC=golang-dev
http://codereview.appspot.com/4438042
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go