5g: fix out of registers bug
5g: fix out of registers bug

Same fix as 6g, tripped by the 6g test case.

R=ken2
CC=golang-dev
http://codereview.appspot.com/5651074
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go