don't crash if ALTER TABLE fails and a long transaction blocks MDL upgrade

mysql-test/suite/heap/heap.result

@@ -398,9 +398,10 @@ qq
 *a       *a*a       *
 *a        *a*a        *
 *a         *a*a         *
+flush tables;
 explain select * from t1 where v='a';
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	ref	v	v	13	const	10	Using where
+1	SIMPLE	t1	ref	v	v	13	const	9	Using where
 select v,count(*) from t1 group by v limit 10;
 v	count(*)
 a
	1