go.crypto/ssh: fix messages_test.go on 32-bit platforms.
go.crypto/ssh: fix messages_test.go on 32-bit platforms.

LGTM=dave
R=agl, dave
CC=golang-codereviews
https://codereview.appspot.com/88060043

Committer: Adam Langley <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

xman1101/terminal,drhax9908/go.crypto,xman1101/terminal,drhax9908/go.crypto