LP #1660628: -Wmaybe-uninitialized sql_planner.cc compilation warnings

**Reported in Launchpad by Laurynas Biveinis last update 23-01-2018 14:12:19

Copy of https://bugs.mysql.com/bug.php?id=77637:

/root/mysql-5.6.25/sql/sql_planner.cc: In member function void Optimize_table_order::best_access_path(JOIN_TAB*, table_map, uint, bool, double, POSITION*, POSITION*):
/root/mysql-5.6.25/sql/sql_planner.cc:431: warning: loose_scan_opt.Loose_scan_opt::best_loose_scan_start_key may be used uninitialized in this function
/root/mysql-5.6.25/sql/sql_planner.cc:431: warning: loose_scan_opt.Loose_scan_opt::best_max_loose_keypart may be used uninitialized in this function
/root/mysql-5.6.25/sql/sql_planner.cc:431: warning: loose_scan_opt.Loose_scan_opt::best_loose_scan_records may be used uninitialized in this function
/root/mysql-5.6.25/sql/sql_planner.cc:431: warning: loose_scan_opt.Loose_scan_opt::best_loose_scan_key may be used uninitialized in this function
/root/mysql-5.6.25/sql/sql_planner.cc:431: warning: loose_scan_opt.Loose_scan_opt::quick_max_loose_keypart may be used uninitialized in this function