Continuation of "Implemented a counter within Item_sum_sum" a few commits before

Query result had 0 where it should have had NULLs:
- Make Item_window_func::val* functions honor NULL-handling conventions:
  1. set null_value to indicate whether we've returned a NULL value
  2. val_str and val_decimal should return NULL pointer when they're
     returning SQL NULL value.

Fix assertion failure when sending results to network.
- The assert was due to window func returing SQL NULL despite having
  maybe_null=false
- Fixed by settting Item_window_func::maybe_null correctly in fix_fields

mysql-test/r/win_sum.result

@@ -24,19 +24,19 @@ select pk, a, b, sum(b) over (partition by a order by pk ROWS BETWEEN 1 PRECEDIN
 sum(c) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
 from t1;
 pk	a	b	sum(b) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)	sum(c) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
-101	0	10	20	3.2
-102	0	10	20	3.2
-103	1	10	20	7.199999999999999
-104	1	10	20	7.199999999999999
-105	2	20	40	13.2
-106	2	20	60	21.35
-107	2	20	50	20.35
-108	2	10	30	13.250000000000002
-109	4	20	40	19.3
-110	4	20	40	19.3
-111	5	NULL	1	23.4
-112	5	1	1	36.75
-113	5	NULL	1	40.1
-114	5	NULL	0	43.5
-115	5	NULL	0	30.15
+101	0	10	0	3.2
+102	0	10	0	3.2
+103	1	10	0	7.199999999999999
+104	1	10	0	7.199999999999999
+105	2	20	0	13.2
+106	2	20	0	21.35
+107	2	20	0	20.35
+108	2	10	0	13.250000000000002
+109	4	20	0	19.3
+110	4	20	0	19.3
+111	5	NULL	0	23.4
+112	5	1	0	36.75
+113	5	NULL	0	40.1
+114	5	NULL	NULL	43.5
+115	5	NULL	NULL	30.15
 drop table t1;

sql/item_windowfunc.cc

@@ -59,6 +59,7 @@ Item_window_func::fix_fields(THD *thd, Item **ref)
   fix_length_and_dec();
 
   max_length= window_func->max_length;
+  maybe_null= window_func->maybe_null;
 
   fixed= 1;
   force_return_blank= true;

sql/item_windowfunc.h

@@ -481,37 +481,91 @@ class Item_window_func : public Item_result_field
 
   double val_real() 
   {
+    double res;
     if (force_return_blank)
-      return 0.0;
-    return read_value_from_result_field? result_field->val_real() :
-                                         window_func->val_real();
+    {
+      res= 0.0;
+      null_value= false;
+    }
+    else if (read_value_from_result_field)
+    {
+      res= result_field->val_real();
+      null_value= false;
+    }
+    else
+    {
+      res= window_func->val_real();
+      null_value= window_func->null_value;
+    }
   }
 
   longlong val_int()
   {
+    longlong res;
     if (force_return_blank)
-      return 0;
-    return read_value_from_result_field? result_field->val_int() : 
-                                          window_func->val_int(); 
+    {
+      res= 0;
+      null_value= false;
+    }
+    else if (read_value_from_result_field)
+    {
+      res= result_field->val_int();
+      null_value= result_field->is_null();
+    }
+    else
+    {
+      res= window_func->val_int();
+      null_value= window_func->null_value;
+    }
+    return res;
   }
 
   String* val_str(String* str)
   {
+    String *res;
     if (force_return_blank)
-      return str;
-    return read_value_from_result_field? result_field->val_str(str) : 
-                                         window_func->val_str(str);
+    {
+      null_value= false;
+      str->length(0);
+      res= str;
+    }
+    else if (read_value_from_result_field)
+    {
+      if ((null_value= result_field->is_null()))
+        res= NULL;
+      else
+        res= result_field->val_str(str);
+    }
+    else
+    {
+      res= window_func->val_str(str);
+      null_value= window_func->null_value;
+    }
+    return res;
   }
 
   my_decimal* val_decimal(my_decimal* dec)
-  { 
+  {
+    my_decimal *res;
     if (force_return_blank)
     {
       my_decimal_set_zero(dec);
-      return dec;
+      null_value= false;
+      res= dec;
+    }
+    else if (read_value_from_result_field)
+    {
+      if ((null_value= result_field->is_null()))
+        res= NULL;
+      else
+        res= result_field->val_decimal(dec);
+    }
+    else
+    {
+      res= result_field->val_decimal(dec);
+      null_value= window_func->null_value;
     }
-    return read_value_from_result_field? result_field->val_decimal(dec) : 
-                                         window_func->val_decimal(dec);
+    return res;
   }
 
   void split_sum_func(THD *thd, Ref_ptr_array ref_pointer_array,