Fix another bug in dense_rank.

When ordering by a column and partitioning by another, we must reset the
peer_tracker for dense_rank, regardless if the value for the order
column changes or not.

Example:
select a, b, dense_rank() over (partition by b order by a)
a  |  b  |  dense_rank
----------------------
1  | p1  |           1
2  | p1  |           2
2  | p2  |           1   // Here, without this fix we returned 0.
2  | p2  |           2   // And 1 here.

mysql-test/r/win_rank.result

@@ -40,11 +40,11 @@ pk	a	b	rank	dense_rank
 3	1	10	3	2
 4	1	10	3	2
 8	2	10	5	3
-5	2	20	1	0
-6	2	20	1	0
-7	2	20	1	0
-9	4	20	4	1
-10	4	20	4	1
+5	2	20	1	1
+6	2	20	1	1
+7	2	20	1	1
+9	4	20	4	2
+10	4	20	4	2
 drop table t1;
 #
 # Test with null values in the table.

sql/item_windowfunc.cc

@@ -185,8 +185,11 @@ void Item_sum_dense_rank::setup_window_func(THD *thd, Window_spec *window_spec)
 
 bool Item_sum_dense_rank::add()
 {
-  if (peer_tracker.check_if_next_group())
+  if (peer_tracker.check_if_next_group() || first_add)
+  {
+    first_add= false;
     dense_rank++;
+  }
 
   return false;
 }

sql/item_windowfunc.h

@@ -213,7 +213,9 @@ class Item_sum_rank: public Item_sum_int
 class Item_sum_dense_rank: public Item_sum_int
 {
   longlong dense_rank;
+  bool first_add;
   Group_bound_tracker peer_tracker;
+ public:
   /*
      XXX(cvicentiu) This class could potentially be implemented in the rank
      class, with a switch for the DENSE case.
@@ -221,6 +223,7 @@ class Item_sum_dense_rank: public Item_sum_int
   void clear()
   {
     dense_rank= 0;
+    first_add= true;
   }
   bool add();
   void update_field() {}
@@ -229,9 +232,8 @@ class Item_sum_dense_rank: public Item_sum_int
     return dense_rank;
   }
 
- public:
   Item_sum_dense_rank(THD *thd)
-    : Item_sum_int(thd), dense_rank(0) {}
+    : Item_sum_int(thd), dense_rank(0), first_add(true) {}
   enum Sumfunctype sum_func () const
   {
     return DENSE_RANK_FUNC;