oracle: add length check o.MainCfg.NeoFS.Nodes

Prevent the risk of a division by zero error when accessing the
`o.MainCfg.NeoFS.Nodes[index]` array.

Close #3419

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

pkg/services/oracle/request.go

@@ -157,6 +157,11 @@ func (o *Oracle) processRequest(priv *keys.PrivateKey, req request) error {
 				resp.Code = transaction.Error
 			}
 		case neofs.URIScheme:
+			if len(o.MainCfg.NeoFS.Nodes) == 0 {
+				o.Log.Warn("No NeoFS nodes configured", zap.String("url", req.Req.URL))
+				resp.Code = transaction.Error
+				break
+			}
 			ctx, cancel := context.WithTimeout(context.Background(), o.MainCfg.NeoFS.Timeout)
 			defer cancel()
 			index := (int(req.ID) + incTx.attempts) % len(o.MainCfg.NeoFS.Nodes)