go.crypto/nacl/secretbox: fix append behaviour.

secretbox is supposed to append to the given slice, like hash.Sum() and
append(). However, I had a complete brain-fart: I've really no idea what
I was thinking of at the time.

R=golang-dev, sfrithjof, rsc
CC=agl, golang-dev
https://golang.org/cl/10099043
2 files changed