Use ii functions to resolve search results by nested index search

It fixes the following test broken by
89c5576a310aa86783804f959285874ac388d117:

  * test/command/suite/select/index/nested/by_column/match/and.test

Detail:

    [30] select/index/nested/by_column/match
      and                                                           1.0461s [failed]
    ================================================================================
    --- (expected)
    +++ (actual)
    @@ -29,35 +29,30 @@
      select Files   --filter '_key @^ "/home/" && owner.hobby @ "programming"'   --output_columns '_key, owner, owner.hobby'
      [
        [
          0,
          0.0,
          0.0
        ],
        [
          [
            [
    ?         1
    ?         0
            ],
            [
              [
                "_key",
                "ShortText"
              ],
              [
                "owner",
                "Users"
              ],
              [
                "owner.hobby",
                "ShortText"
              ]
    -       ],
    -       [
    -         "/home/alice/.zshrc",
    -         "Alice",
    -         "programming"
            ]
          ]
        ]
      ]

See also: https://travis-ci.org/groonga/groonga/jobs/16860080