On shutdown feedback was sending a short report without creating
a THD. At that point current_thd was pointing to the already
destroyed THD from the previous full report.

backport from 10.1:
  commit bfe703a4
  Author: Sergei Golubchik <serg@mariadb.org>
  Date:   Tue Feb 3 18:19:56 2015 +0100

      don't let current_thd to point to a destroyed THD