[bug] function RegistryLogin in client package has incorrect logic

[allencloud]

Ⅰ. Issue Description

When I was reviewing code in #1015, I found in master, the code following has incorrect logic:

// RegistryLogin authenticates the server with a given registry to login.
func (client *APIClient) RegistryLogin(ctx context.Context, auth *types.AuthConfig) (*types.AuthResponse, error) {
	resp, err := client.post(ctx, "/auth", nil, auth, nil)
	if err != nil || resp.StatusCode == http.StatusUnauthorized {
		return nil, err
	}

	authResp := &types.AuthResponse{}
	err = decodeBody(authResp, resp.Body)
	ensureCloseReader(resp)

	return authResp, err

In code above, when err == nil, and resp.StatusCode == http.StatusUnauthorized, code will return nil, nil. I don't think this is reasonable. @Ace-Tang