MDEV-20799 DROP Virtual Column crashes MariaDB

use the correct table for evaluating virtual columns in the
InnoDB ALTER TABLE.

mysql-test/suite/vcol/r/vcol_keys_innodb.result

@@ -269,3 +269,18 @@ index(col_char,vcol_blob(64))
 insert ignore into t1 (pk) values (1),(2);
 update t1 set col_char = 'foo' where pk = 1;
 drop table t1;
+create table t1 (
+id int not null primary key,
+a varchar(200),
+b varchar(200),
+c int,
+va char(200)  generated always as (ucase(a)) virtual,
+vb char(200) generated always as (ucase(b)) virtual,
+key (c,va,vb)
+) engine=innodb;
+insert t1 (id,a,c) select seq,seq,seq from seq_1_to_330;
+select IF(@@innodb_sort_buffer_size < count(*)*200, 'GOOD', 'WRONG SIZE') from t1;
+IF(@@innodb_sort_buffer_size < count(*)*200, 'GOOD', 'WRONG SIZE')
+GOOD
+alter table t1 drop column va;
+drop table t1;

mysql-test/suite/vcol/t/vcol_keys_innodb.opt

+--innodb-sort-buffer-size=64k

mysql-test/suite/vcol/t/vcol_keys_innodb.test

@@ -117,3 +117,21 @@ create table t1 (
 insert ignore into t1 (pk) values (1),(2);
 update t1 set col_char = 'foo' where pk = 1;
 drop table t1;
+
+#
+# MDEV-20799 DROP Virtual Column crashes MariaDB
+#
+--source include/have_sequence.inc
+create table t1 (
+  id int not null primary key,
+  a varchar(200),
+  b varchar(200),
+  c int,
+  va char(200)  generated always as (ucase(a)) virtual,
+  vb char(200) generated always as (ucase(b)) virtual,
+  key (c,va,vb)
+) engine=innodb;
+insert t1 (id,a,c) select seq,seq,seq from seq_1_to_330;
+select IF(@@innodb_sort_buffer_size < count(*)*200, 'GOOD', 'WRONG SIZE') from t1;
+alter table t1 drop column va;
+drop table t1;

storage/innobase/row/row0merge.cc

@@ -2568,7 +2568,7 @@ row_merge_read_clustered_index(
 						buf, fts_index, old_table,
 						new_table, psort_info, row, ext,
 						&doc_id, conv_heap,
-						&err, &v_heap, table, trx)))) {
+						&err, &v_heap, eval_table, trx)))) {
 					/* An empty buffer should have enough
 					room for at least one record. */
 					ut_error;