index_column_diff: fix a bug that the number of elements is wrong
index_column_diff: fix a bug that the number of elements is wrong

GitHub: fix #1010

It causes broken MessagePack output.

Reported by yagisumi. Thanks!!!

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