Shame on me: I fixed the same bug in 6l in 8691fcc6a66e
(https://golang.org/cl/2609041) and neglected
to look at 5l and 8l to see if they were affected.

On the positive side, the check I added in that CL is the
one that detected this bug.

Fixes #1457.

R=ken2
CC=golang-dev
https://golang.org/cl/3981052