Fix create_background_thd()

Allow the caller to have current_thd. Also do not store
PSI_CALL_get_thread() in the new THD, it is a thread local storage variable
that can become invalid any time, we do not control the lifetime of the
caller's thread.

sql/sql_class.cc

@@ -4894,11 +4894,13 @@ void destroy_thd(MYSQL_THD thd)
 extern "C" pthread_key(struct st_my_thread_var *, THR_KEY_mysys);
 MYSQL_THD create_background_thd()
 {
-  DBUG_ASSERT(!current_thd);
+  auto save_thd = current_thd;
+  set_current_thd(nullptr);
+
   auto save_mysysvar= pthread_getspecific(THR_KEY_mysys);
 
   /*
-    Allocate new mysys_var specifically this THD,
+    Allocate new mysys_var specifically new THD,
     so that e.g safemalloc, DBUG etc are happy.
   */
   pthread_setspecific(THR_KEY_mysys, 0);
@@ -4906,7 +4908,8 @@ MYSQL_THD create_background_thd()
   auto thd_mysysvar= pthread_getspecific(THR_KEY_mysys);
   auto thd= new THD(0);
   pthread_setspecific(THR_KEY_mysys, save_mysysvar);
-  thd->set_psi(PSI_CALL_get_thread());
+  thd->set_psi(nullptr);
+  set_current_thd(save_thd);
 
   /*
     Workaround the adverse effect of incrementing thread_count