Fix the complaint from safemalloc about leaked 8 bytes.

When mysql_update() uses direct update, it will still create a quick select
(doesn't make much sense, does it?)
Quick select will be initialized, which will call ha_xpand::index_init
which will allocate ha_xpand::scan_fields.

Then, ha_xpand::index_end will be called but it will fail to free scan_fields.