ipn/ipnlocal: prevent changing serve config if conf.Locked

This adds a check to prevent changes to ServeConfig if tailscaled
is run with a Locked config.

Missed in 1fc3573446.

Updates #1412

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnlocal/serve.go

@@ -245,6 +245,9 @@ func (b *LocalBackend) setServeConfigLocked(config *ipn.ServeConfig, etag string
 	if config.IsFunnelOn() && prefs.ShieldsUp() {
 		return errors.New("Unable to turn on Funnel while shields-up is enabled")
 	}
+	if b.isConfigLocked_Locked() {
+		return errors.New("can't reconfigure tailscaled when using a config file; config file is locked")
+	}
 
 	nm := b.netMap
 	if nm == nil {