go.crypto/ssh: prevent concurrent reads and concurrent writes over the same agent connection

minor fix for v01 cert parsing when algo is not supported

R=golang-dev, agl, dave
CC=golang-dev
https://golang.org/cl/6116052

ssh/certs.go

@@ -78,6 +78,7 @@ func parseOpenSSHCertV01(in []byte, algo string) (out *OpenSSHCertV01, rest []by
 		}
 		cert.Key = dsaPubKey
 	default:
+		ok = false
 		return
 	}