MDEV-22633 Assertion failed in prepare_inplace_alter_table_dict

prepare_inplace_alter_table_dict(): In the error handling, relax
a debug assertion for the case that we did not execute
dict_stats_wait_bg_to_stop_using_table() yet.

storage/innobase/handler/handler0alter.cc

@@ -4404,6 +4404,7 @@ prepare_inplace_alter_table_dict(
 	create_table_info_t info(ctx->prebuilt->trx->mysql_thd, altered_table,
 				 ha_alter_info->create_info, NULL, NULL,
 				 srv_file_per_table);
+	ut_d(bool stats_wait = false);
 
 	if (num_fts_index > 1) {
 		my_error(ER_INNODB_FT_LIMIT, MYF(0));
@@ -4477,6 +4478,7 @@ prepare_inplace_alter_table_dict(
 	XXX what may happen if bg stats opens the table after we
 	have unlocked data dictionary below? */
 	dict_stats_wait_bg_to_stop_using_table(user_table, ctx->trx);
+	ut_d(stats_wait = true);
 
 	online_retry_drop_indexes_low(ctx->new_table, ctx->trx);
 
@@ -5116,7 +5118,8 @@ prepare_inplace_alter_table_dict(
 		/* n_ref_count must be 1, because purge cannot
 		be executing on this very table as we are
 		holding dict_operation_lock X-latch. */
-		DBUG_ASSERT(user_table->get_ref_count() == 1 || ctx->online);
+		ut_ad(!stats_wait || ctx->online
+		      || user_table->get_ref_count() == 1);
 
 		online_retry_drop_indexes_with_trx(user_table, ctx->trx);
 	} else {