MDEV-21564 Assert in trx_purge_add_update_undo_to_history during shutdown

- dict_table_try_drop_aborted() can generate undo logs. So it should
be avoided after shutdown of background threads.

storage/innobase/dict/dict0dict.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1996, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -604,7 +604,10 @@ dict_table_close(
 
 		mutex_exit(&dict_sys->mutex);
 
-		if (drop_aborted) {
+		/* dict_table_try_drop_aborted() can generate undo logs.
+		So it should be avoided after shutdown of background
+		threads */
+		if (drop_aborted && !srv_undo_sources) {
 			dict_table_try_drop_aborted(NULL, table_id, 0);
 		}
 	}

storage/xtradb/dict/dict0dict.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1996, 2017, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -604,7 +604,10 @@ dict_table_close(
 
 		mutex_exit(&dict_sys->mutex);
 
-		if (drop_aborted) {
+		/* dict_table_try_drop_aborted() can generate undo logs.
+		So it should be avoided after shutdown of background
+		threads */
+		if (drop_aborted && srv_undo_sources) {
 			dict_table_try_drop_aborted(NULL, table_id, 0);
 		}
 	}