Fix a bug that grn_index_cursor_next() returns NULL unexpectedly

It causes mroonga's bug.

Index may have garbages after deleting entries. For the index entry,
grn_ii_cursor_next() returns NULL. It should be ignored in
grn_index_cursor_next(). But it returns NULL for the case without this
change.

[groonga-dev,01192]

Reported by b senboku. Thanks!!!