The root cause of the bug is in `spider_db_mbase_util::open_item_func()`.
The function handles an instance of the `Item_func` class based on its
`Item_func::Functype`.
The `Functype` of `CASE WHEN ... THEN` is `CASE_SEARCHED_FUNC`.
However, the Spider SE doesn't recognize this `Functype` because
`CASE_SEARCHED_FUNC` is newly added by 4de0d92. This results in the wrong
handling of `CASE WHEN ... THEN`.
The above also applies to `CASE_SIMPLE_FUNC`.
