MDEV-5461 Assertion `length <= column->length' fails in write_block_record with functions in select list, GROUP BY, ORDER BY

Old code in create_tmp_table(), that created an extra one-byte field (recinfo)
before every NULL-able grouping field (Field) in the tmp table, did not actually work.
Because the matching code in end_update(), that was supposed to update this byte,
was using a wrong offset, updating the first byte of the Field, not a byte before it.
Normally this wasn't an issue, because the Field value (written later in end_update)
was overwriting this byte anyway. But in this bug the Field was Field_null, with zero
length, so end_update() was overwriting the first byte of the following field.
And the following field was not-nullable constant, which was stored only once in
create_tmp_table and never updated later.

Fixed by removing the code that didn't do any useful work anyway.

1 files changed, 7 insertions, 0 deletions

diff --git a/mysql-test/t/group_by_null.test b/mysql-test/t/group_by_null.test
new file mode 100644
index 00000000000..b3fa2a003ec
--- /dev/null
+++ b/mysql-test/t/group_by_null.test
@@ -0,0 +1,7 @@
+#
+# MDEV-5461 Assertion `length <= column->length' fails in write_block_record with functions in select list, GROUP BY, ORDER BY
+#
+create table t1 (a int);
+insert into t1 values (1),(2);
+select max('foo') from t1 group by values(a), extractvalue('bar','qux') order by "v";
+drop table t1;