[#142] Fix multipart-objects download

Signed-off-by: Nikita Zinkevich <n.zinkevich@yadro.com>

internal/handler/multipart.go

@@ -3,6 +3,7 @@ package handler
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"strconv"
@@ -76,8 +77,14 @@ func (h *Handler) getPayload(p getPayloadParams) (io.ReadCloser, uint64, error)
 	if !ok {
 		return p.obj.Payload, p.obj.Header.PayloadSize(), nil
 	}
-	cid, _ := p.obj.Header.ContainerID()
-	oid, _ := p.obj.Header.ID()
+	cid, ok := p.obj.Header.ContainerID()
+	if !ok {
+		return nil, 0, errors.New("no container id set")
+	}
+	oid, ok := p.obj.Header.ID()
+	if !ok {
+		return nil, 0, errors.New("no object id set")
+	}
 	size, err := strconv.ParseUint(sizeValue, 10, 64)
 	if err != nil {
 		return nil, 0, err