MDEV-18190 A table with variable-length PRIMARY KEY is unaccessible after instant DROP COLUMN

btr_cur_instant_init_low(): Fix the typo in the computation
of the trx_id_offset of a metadata record.

mysql-test/suite/innodb/r/instant_drop.result

@@ -154,6 +154,9 @@ alter table t2 add column f6 char(100) default repeat('a', 99), algorithm=instan
 create table t3(f1 int, f2 int not null)engine=innodb;
 insert into t3 values(1, 2);
 alter table t3 drop column f2, add column f3 int default 1, add column f4 int default 4, algorithm=instant;
+create table t4(a varchar(1), b int, c int, primary key(a,b))engine=innodb;
+insert into t4 values('4',5,6);
+alter table t4 drop column c;
 select * from t1;
 f5	f3
 10	19
@@ -185,7 +188,16 @@ select * from t3;
 f1	f3	f4
 1	1	4
 alter table t3 add column f5 char(100) default repeat('a', 99), algorithm=instant;
+select * from t4;
+a	b
+4	5
+alter table t4 add column d varchar(5) default 'fubar';
+insert into t4 values('',0,'snafu');
 select * from t3;
 f1	f3	f4	f5
 1	1	4	aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
-drop table t1,t2,t3;
+select * from t4;
+a	b	d
+	0	snafu
+4	5	fubar
+drop table t1,t2,t3,t4;

mysql-test/suite/innodb/t/instant_drop.test

@@ -80,6 +80,10 @@ create table t3(f1 int, f2 int not null)engine=innodb;
 insert into t3 values(1, 2);
 alter table t3 drop column f2, add column f3 int default 1, add column f4 int default 4, algorithm=instant;
 
+create table t4(a varchar(1), b int, c int, primary key(a,b))engine=innodb;
+insert into t4 values('4',5,6);
+alter table t4 drop column c;
+
 --source include/restart_mysqld.inc
 select * from t1;
 alter table t1 add column f6 int default 9,drop column f5, algorithm = instant;
@@ -94,6 +98,11 @@ show create table t2;
 
 select * from t3;
 alter table t3 add column f5 char(100) default repeat('a', 99), algorithm=instant;
+
+select * from t4;
+alter table t4 add column d varchar(5) default 'fubar';
+insert into t4 values('',0,'snafu');
 --source include/restart_mysqld.inc
 select * from t3;
-drop table t1,t2,t3;
+select * from t4;
+drop table t1,t2,t3,t4;

storage/innobase/btr/btr0cur.cc

@@ -486,7 +486,7 @@ static dberr_t btr_cur_instant_init_low(dict_index_t* index, mtr_t* mtr)
 			always written with zero length. The DB_TRX_ID will
 			start right after any fixed-length columns. */
 			for (uint i = index->n_uniq; i--; ) {
-				trx_id_offset += index->fields[0].fixed_len;
+				trx_id_offset += index->fields[i].fixed_len;
 			}
 		}