MDEV-9935: Window functions: assertion failure with empty OVER () clause

Make window functions work with an empty over clause by forcing
a sort on the first column of the current join_tab. This is a temporary
fix until we get window functions to work with big tables.

mysql-test/t/win_empty_over.test

@@ -0,0 +1,66 @@
+create table t1 (
+  pk int primary key,
+  a int,
+  b int,
+  c char(10),
+  d decimal(10, 3),
+  e real
+);
+
+insert into t1 values
+( 1, 0, 1,    'one',    0.1,  0.001),
+( 2, 0, 2,    'two',    0.2,  0.002),
+( 3, 0, 3,    'three',  0.3,  0.003),
+( 4, 1, 2,    'three',  0.4,  0.004),
+( 5, 1, 1,    'two',    0.5,  0.005),
+( 6, 1, 1,    'one',    0.6,  0.006),
+( 7, 2, NULL, 'n_one',  0.5,  0.007),
+( 8, 2, 1,    'n_two',  NULL, 0.008),
+( 9, 2, 2,    NULL,     0.7,  0.009),
+(10, 2, 0,    'n_four', 0.8,  0.010),
+(11, 2, 10,   NULL,     0.9,  NULL);
+
+select pk, row_number() over () from t1;
+explain FORMAT=JSON select pk, row_number() over () from t1;
+explain FORMAT=JSON select row_number() over (), pk from t1;
+
+select row_number() over () from (select 4) as t;
+
+--sorted_result
+select min(a) over (), max(a) over (), a, row_number() over ()
+from t1
+where a = 0;
+
+--sorted_result
+select a, min(a) over (), max(a) over (), row_number() over ()
+from t1
+where a = 0;
+
+--sorted_result
+select min(a) over () + 1, max(a) over (), row_number() over ()
+from t1
+where a = 0;
+
+--sorted_result
+select min(a) over () + a, max(a) over (), row_number() over ()
+from t1
+where a = 1;
+
+--sorted_result
+select a + min(a) over (), max(a) over (), row_number() over ()
+from t1
+where a = 1;
+
+select a + min(a) over () from t1 where a = 1;
+
+create view win_view
+as (select a, min(a) over () from t1 where a = 1);
+select * from win_view;
+drop view win_view;
+
+create view win_view
+as (select a, max(a + 1) over () from t1 where a = 1);
+select * from win_view;
+drop view win_view;
+
+drop table t1;