branches/zip: row_scan_and_check_index(): Initialize prebuilt->index_usable. This should have been done in r4631.  Spotted by Michael.
branches/zip: row_scan_and_check_index(): Initialize prebuilt->index_usable.
This should have been done in r4631.  Spotted by Michael.

flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,flynn1973/mariadb-aix,slanterns/server