MDEV-15235: Assertion `length > 0' failed in create_ref_for_key

The issue is that we are creating a materialised table with key of length 0 which is incorrect, we should
disable materialisation for such a case.

mysql-test/r/subselect_mat.result

@@ -2642,3 +2642,18 @@ a	b	sq
 4	4	1
 4	2	1
 drop table t1, t2;
+#
+# MDEV-15235: Assertion `length > 0' failed in create_ref_for_key
+#
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (f CHAR(1));
+INSERT INTO t2 VALUES ('a'),('b');
+explain
+SELECT * FROM t2 WHERE f IN ( SELECT LEFT('foo',0) FROM t1 ORDER BY 1 );
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	Using where
+2	DEPENDENT SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	
+SELECT * FROM t2 WHERE f IN ( SELECT LEFT('foo',0) FROM t1 ORDER BY 1 );
+f
+DROP TABLE t1, t2;

mysql-test/t/subselect_mat.test

@@ -254,3 +254,16 @@ SELECT a, b, (a, b) NOT IN (SELECT a, b FROM t2) as sq
 FROM t1;
 
 drop table t1, t2;
+
+--echo #
+--echo # MDEV-15235: Assertion `length > 0' failed in create_ref_for_key
+--echo #
+
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (f CHAR(1));
+INSERT INTO t2 VALUES ('a'),('b');
+explain
+SELECT * FROM t2 WHERE f IN ( SELECT LEFT('foo',0) FROM t1 ORDER BY 1 );
+SELECT * FROM t2 WHERE f IN ( SELECT LEFT('foo',0) FROM t1 ORDER BY 1 );
+DROP TABLE t1, t2;

sql/opt_subselect.cc

@@ -873,8 +873,10 @@ bool subquery_types_allow_materialization(Item_in_subselect *in_subs)
      Make sure that create_tmp_table will not fail due to too long keys.
      See MDEV-7122. This check is performed inside create_tmp_table also and
      we must do it so that we know the table has keys created.
+     Make sure that the length of the key for the temp_table is atleast
+     greater than 0.
   */
-  if (total_key_length > tmp_table_max_key_length() ||
+  if (!total_key_length || total_key_length > tmp_table_max_key_length() ||
       elements > tmp_table_max_key_parts())
     DBUG_RETURN(FALSE);