vector_slice: fix a bug that reference vector can't be detected
vector_slice: fix a bug that reference vector can't be detected

It's introduced by 6d460140176dfb5c348820963bda11dbb114c5e9.

naoa/groonga,naoa/groonga,komainu8/groonga,komainu8/groonga,komainu8/groonga,cosmo0920/groonga,naoa/groonga,cosmo0920/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,cosmo0920/groonga,cosmo0920/groonga,cosmo0920/groonga,kenhys/groonga,kenhys/groonga,cosmo0920/groonga,groonga/groonga,naoa/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,groonga/groonga,naoa/groonga,naoa/groonga,komainu8/groonga,naoa/groonga,cosmo0920/groonga,groonga/groonga,cosmo0920/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga