tls: fix vet failure at tip
tls: fix vet failure at tip

paulbellamy/go-dockerclient,davidstack/go-dockerclient,tarsisazevedo/go-dockerclient,tarsisazevedo/go-dockerclient,ybogdanov/go-dockerclient,csrwng/go-dockerclient,csrwng/go-dockerclient,davidstack/go-dockerclient,fsouza/go-dockerclient,ybogdanov/go-dockerclient