MDEV-21787 Alter table failure tries to access uninitialized column

- Forget to record .result for instant_alter_limit 16k test case

mysql-test/suite/innodb/r/instant_alter_limit,16k.rdiff

+--- instant_alter_limit.result	2020-05-26 18:01:27.377946439 +0530
++++ instant_alter_limit,16k.reject	2020-05-26 20:14:38.452463919 +0530
+@@ -45,3 +45,10 @@
+ instants
+ 502
+ DROP TABLE t;
++#
++# MDEV-21787 Alter table failure tries to access uninitialized column
++#
++CREATE TABLE t1(f1 INT PRIMARY KEY, f2 TEXT GENERATED ALWAYS AS (SUBSTR(f4, 1, 400)), f3 VARCHAR(500), f4 TEXT)ENGINE=InnoDB ROW_FORMAT=Compact;
++ALTER TABLE t1 ADD UNIQUE KEY (f2(9));
++ALTER TABLE t1 ADD COLUMN f5 TEXT, ALGORITHM=INPLACE;
++DROP TABLE t1;