grn_arrow_load: fix a crash bug
grn_arrow_load: fix a crash bug

load command isn't related. Rroonga's Table#load_arrow users are only
affected.

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