MDEV-11704 InnoDB: Failing assertion: dfield_is_null(dfield2) || dfield2->data

relax innodb assertion, because Field_blob::store() clearly says
that a data pointer can be zero if the length is zero.

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

@@ -258,3 +258,14 @@ insert into t1 (col_varchar,col_int,col_datetime,col_time,col_blob,col_bit,col_y
 ('bar',6,'1900-01-01 00:00:00','00:00:00','bar',b'10011000001101011000101',1985,'b',0.7,'','2028-04-06','1971-01-01 00:00:00');
 alter table t1 add index(vcol_datetime);
 drop table t1;
+create table t1 (
+pk int,
+col_blob mediumtext not null default '',
+vcol_blob tinyblob as (col_blob) virtual,
+col_char char(22) null,
+primary key(pk),
+index(col_char,vcol_blob(64))
+) engine=innodb;
+insert ignore into t1 (pk) values (1),(2);
+update t1 set col_char = 'foo' where pk = 1;
+drop table t1;

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

@@ -101,3 +101,19 @@ insert into t1 (col_varchar,col_int,col_datetime,col_time,col_blob,col_bit,col_y
 
 alter table t1 add index(vcol_datetime);
 drop table t1;
+
+
+#
+# MDEV-11704 InnoDB: Failing assertion: dfield_is_null(dfield2) || dfield2->data
+#
+create table t1 (
+ pk int,
+ col_blob mediumtext not null default '',
+ vcol_blob tinyblob as (col_blob) virtual,
+ col_char char(22) null,
+ primary key(pk),
+ index(col_char,vcol_blob(64))
+ ) engine=innodb;
+insert ignore into t1 (pk) values (1),(2);
+update t1 set col_char = 'foo' where pk = 1;
+drop table t1;

sql/table.cc

@@ -6243,7 +6243,7 @@ void TABLE::mark_columns_needed_for_delete()
 
 void TABLE::mark_columns_needed_for_update()
 {
-  DBUG_ENTER("mark_columns_needed_for_update");
+  DBUG_ENTER("TABLE::mark_columns_needed_for_update");
   bool need_signal= false;
 
   mark_columns_per_binlog_row_image();

storage/innobase/row/row0row.cc

@@ -129,7 +129,8 @@ row_build_index_entry_low(
 			ut_ad(v_col->v_pos < dtuple_get_n_v_fields(row));
 			dfield2 = dtuple_get_nth_v_field(row, v_col->v_pos);
 
-			ut_ad(dfield_is_null(dfield2) || dfield2->data);
+			ut_ad(dfield_is_null(dfield2) ||
+			      dfield_get_len(dfield2) == 0 || dfield2->data);
 		} else {
 			dfield2 = dtuple_get_nth_field(row, col_no);
 			ut_ad(dfield_get_type(dfield2)->mtype == DATA_MISSING