go.crypto/bcrypt: fix interger overflow for cost == 31 Fixes golang/go#4803.
go.crypto/bcrypt: fix interger overflow for cost == 31
Fixes golang/go#4803.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7329043

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