strings: fix off-by-one error in test
strings: fix off-by-one error in test

Previously it would panic because of out-of-bound access
if s1 is longer than s2.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/90110043

Committer: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old