go.crypto/ssh/test: don't kill process if it was never started
go.crypto/ssh/test: don't kill process if it was never started

This case arises if s.cmd.Start fails when called by
server.Dial.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/6821097

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