8l: correct test for sp == top of stack in 8l -K code
8l: correct test for sp == top of stack in 8l -K code

R=ken2
CC=golang-dev
http://codereview.appspot.com/1627042
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go