[Graph]: fix a bug in vertex_index_reader.
[Graph]: fix a bug in vertex_index_reader.

We need to use 64bit integer for integer overflow.

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