Killed 'select' query remains in processlist until being killed a second time.

A user running a custom MariaDB 10.2.14 build, #21251 from the 10.2-compatibility branch, finds that certain select queries remain in the processlist until being killed explicitly a second time. At this time no additional adverse effects are noted, but user is concerned because of the locked resource problem observed in the similar MDEV-15214.