x/crypto/ssh: bail early if a server has no auth methods configured.

Change-Id: I58fdfbe00fcc4ca09da9699edcc181cc512feef7
Reviewed-on: https://go-review.googlesource.com/9807
Reviewed-by: JP Sugarbroad <jpsugar@google.com>
Reviewed-by: Adam Langley <agl@golang.org>

ssh/server.go

@@ -168,6 +168,10 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 		return nil, errors.New("ssh: server has no host keys")
 	}
 
+	if !config.NoClientAuth && config.PasswordCallback == nil && config.PublicKeyCallback == nil && config.KeyboardInteractiveCallback == nil {
+		return nil, errors.New("ssh: no authentication methods configured but NoClientAuth is also false")
+	}
+
 	if config.ServerVersion != "" {
 		s.serverVersion = []byte(config.ServerVersion)
 	} else {