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

diff --git a/ssh/certs.go b/ssh/certs.go
index 107fd1a..40cf706 100644
--- a/ssh/certs.go
+++ b/ssh/certs.go
@@ -78,6 +78,7 @@
 		}
 		cert.Key = dsaPubKey
 	default:
+		ok = false
 		return
 	}