[Graph]: fix warning in vertex index reader.
[Graph]: fix warning in vertex index reader.

icoming/FlashGraph,zheng-da/FlashX,zheng-da/FlashX,icoming/FlashX,flashxio/FlashX,zheng-da/FlashX,icoming/FlashGraph,icoming/FlashGraph,icoming/FlashX,icoming/FlashX,icoming/FlashGraph,flashxio/FlashX,flashxio/FlashX,flashxio/FlashX,icoming/FlashGraph,flashxio/FlashX,zheng-da/FlashX,icoming/FlashX,flashxio/FlashX,zheng-da/FlashX,icoming/FlashX