MDEV-24084 Fix race between disconnect and KILL CONNECTION

Prior to this patch, it is possible to access freed memory
(THD::event_scheduler) from tp_post_kill_notification().

With this patch, memory is freed only when THD is no more accessible
from other threads, i.e after it is removed from the thread_list.

sql/threadpool_common.cc

@@ -202,12 +202,11 @@ void tp_callback(TP_connection *c)
 
 error:
   c->thd= 0;
-  delete c;
-
   if (thd)
   {
     threadpool_remove_connection(thd);
   }
+  delete c;
   worker_context.restore();
 }