Fix panic when failing to get DefaultAuthConfig

Commit f32731f902 fixed a potential panic
when an error was returned while trying to get existing credentials.

However, other code paths currently use the result of `GetDefaultAuthConfig()`
even in an error condition; this resulted in a panic, because a `nil` was
returned.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/registry.go

@@ -63,6 +63,9 @@ func RegistryAuthenticationPrivilegedFunc(cli Cli, index *registrytypes.IndexInf
 		indexServer := registry.GetAuthConfigKey(index)
 		isDefaultRegistry := indexServer == ElectAuthServer(context.Background(), cli)
 		authConfig, err := GetDefaultAuthConfig(cli, true, indexServer, isDefaultRegistry)
+		if authConfig == nil {
+			authConfig = &types.AuthConfig{}
+		}
 		if err != nil {
 			fmt.Fprintf(cli.Err(), "Unable to retrieve stored credentials for %s, error: %s.\n", indexServer, err)
 		}

cli/command/registry/login.go

@@ -111,11 +111,12 @@ func runLogin(dockerCli command.Cli, opts loginOptions) error { //nolint: gocycl
 		serverAddress = authServer
 	}
 
-	var err error
-	var authConfig *types.AuthConfig
 	var response registrytypes.AuthenticateOKBody
 	isDefaultRegistry := serverAddress == authServer
-	authConfig, err = command.GetDefaultAuthConfig(dockerCli, opts.user == "" && opts.password == "", serverAddress, isDefaultRegistry)
+	authConfig, err := command.GetDefaultAuthConfig(dockerCli, opts.user == "" && opts.password == "", serverAddress, isDefaultRegistry)
+	if authConfig == nil {
+		authConfig = &types.AuthConfig{}
+	}
 	if err == nil && authConfig.Username != "" && authConfig.Password != "" {
 		response, err = loginWithCredStoreCreds(ctx, dockerCli, authConfig)
 	}