api/types/jsonstream: prevent panic on nil-Error

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

api/types/jsonstream/json_error.go

@@ -8,5 +8,8 @@ type Error struct {
 }
 
 func (e *Error) Error() string {
+	if e == nil {
+		return "<nil>"
+	}
 	return e.Message
 }

api/types/jsonstream/json_error_test.go

@@ -1,13 +1,19 @@
-package jsonstream
+package jsonstream_test
 
 import (
 	"testing"
 
+	"github.com/moby/moby/api/types/jsonstream"
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
 )
 
 func TestError(t *testing.T) {
-	je := Error{404, "Not found"}
+	je := jsonstream.Error{Code: 404, Message: "Not found"}
 	assert.Assert(t, is.Error(&je, "Not found"))
 }
+
+func TestNilError(t *testing.T) {
+	var je *jsonstream.Error
+	assert.Assert(t, is.Error(je, "<nil>"))
+}