Avoid index out of range if len(goldenLines) == len(outLines) + 1

Change-Id: Ic23a85d2b8dd06a615e35a58331e78abe4ad6703
Reviewed-on: https://go-review.googlesource.com/130396
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>