MDEV-18289 - Fix a race between thd_destructor_proxy() startup and

server shutdown.

Prior to adding current thd to the global thd list, initialize
thd->mysys_var->current_cond / thd->mysys_var->current_mutex.

Otherwise thd_destructor_proxy can miss the abort condition, which is set
by the shutdown thread.

storage/innobase/handler/ha_innodb.cc

@@ -317,11 +317,12 @@ thd_destructor_proxy(void *)
 	mysql_cond_init(PSI_NOT_INSTRUMENTED, &thd_destructor_cond, 0);
 
 	st_my_thread_var *myvar= _my_thread_var();
+	myvar->current_mutex = &thd_destructor_mutex;
+	myvar->current_cond = &thd_destructor_cond;
+
 	THD *thd= create_thd();
 	thd_proc_info(thd, "InnoDB shutdown handler");
 
-	myvar->current_mutex = &thd_destructor_mutex;
-	myvar->current_cond = &thd_destructor_cond;
 
 	mysql_mutex_lock(&thd_destructor_mutex);
 	my_atomic_storeptr_explicit(reinterpret_cast<void**>(&srv_running),