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://golang.org/cl/88060043

cheffo/crypto,cmars/tools,zj8487/crypto,Carrotman42/crypto,mattharden/crypto,lucevers/crypto,guaiguaihw/crypto,CodyGuo/crypto,bford/golang-x-crypto,ScriptRock/crypto,golang/crypto,conseweb/golangcrypto,wooutl/golangcrypto,pki-io/crypto,thomaso-mirodin/crypto,a-palchikov/crypto,ScriptRock/crypto,juniorz/crypto,golang/crypto,cmars/tools,tca/crypto,tca/crypto,CodyGuo/crypto,kim-racktop/crypto,pki-io/crypto,golang/crypto,Carrotman42/crypto,mattharden/crypto,thomaso-mirodin/crypto,guaiguaihw/crypto,bford/golang-x-crypto,conseweb/golangcrypto,juniorz/crypto,zj8487/crypto,lucevers/crypto,cmars/tools,cmars/tools,cheffo/crypto,progrium/crypto,progrium/crypto,wooutl/golangcrypto,kim-racktop/crypto,a-palchikov/crypto