vector_find: fix a bug that wrong pvector access
vector_find: fix a bug that wrong pvector access

kenhys/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga