filemanager/manager: surface incomplete prepare-upload response instead of nil-deref

Signed-off-by: SAY-5 <say.apm35@gmail.com>

pkg/filemanager/manager/upload.go

@@ -406,6 +406,12 @@ func (m *manager) updateStateless(ctx context.Context, req *fs.UploadRequest, o
 	if err != nil {
 		return nil, fmt.Errorf("faield to prepare uplaod: %w", err)
 	}
+	// A successful upstream call may still return a response with nil Req or
+	// Session — observed when the master returned 502 mid-prepare (#3441) and
+	// crashed the slave on the next attribute access.
+	if res == nil || res.Req == nil || res.Session == nil {
+		return nil, fmt.Errorf("stateless prepare upload returned an incomplete response")
+	}
 
 	req.Props = res.Req.Props
 	if err := m.Upload(ctx, req, res.Session.Policy, res.Session); err != nil {