Fix 2 more bugs in item_windowfunc.

1. Item_windowfunc must check the nullness of the field if it returns the
   value from the result_field.
2. The decimal value must come from the window_func instead of the
   result_field when not fetching values from the result_field.

Added a more extensive test case that highlights if these bugs occur.
The 2 select statements check behaviour for decimal and real typed
fields respectively.

mysql-test/r/win_sum.result

@@ -19,24 +19,77 @@ insert into t1 values
 (112 , 5, 1, 12.25),
 (113 , 5, NULL, 13.35),
 (114 , 5, NULL, 14.50),
-(115 , 5, NULL, 15.65);
-select 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)
+(115 , 5, NULL, 15.65),
+(116 , 6, 1, NULL),
+(117 , 6, 1, 10),
+(118 , 6, 1, 1.1),
+(119 , 6, 1, NULL),
+(120 , 6, 1, NULL),
+(121 , 6, 1, NULL),
+(122 , 6, 1, 2.2),
+(123 , 6, 1, 20.1),
+(124 , 6, 1, -10.4),
+(125 , 6, 1, NULL),
+(126 , 6, 1, NULL),
+(127 , 6, 1, NULL);
+select pk, a, b, sum(b) 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	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
+pk	a	b	sum(b) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
+101	0	10	20
+102	0	10	20
+103	1	10	20
+104	1	10	20
+105	2	20	40
+106	2	20	60
+107	2	20	50
+108	2	10	30
+109	4	20	40
+110	4	20	40
+111	5	NULL	1
+112	5	1	1
+113	5	NULL	1
+114	5	NULL	NULL
+115	5	NULL	NULL
+116	6	1	2
+117	6	1	3
+118	6	1	3
+119	6	1	3
+120	6	1	3
+121	6	1	3
+122	6	1	3
+123	6	1	3
+124	6	1	3
+125	6	1	3
+126	6	1	3
+127	6	1	2
+select pk, a, c, sum(c) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
+from t1;
+pk	a	c	sum(c) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
+101	0	1.1	3.2
+102	0	2.1	3.2
+103	1	3.1	7.199999999999999
+104	1	4.1	7.199999999999999
+105	2	6.1	13.2
+106	2	7.1	21.35
+107	2	8.15	20.35
+108	2	5.1	13.250000000000002
+109	4	9.15	19.3
+110	4	10.15	19.3
+111	5	11.15	23.4
+112	5	12.25	36.75
+113	5	13.35	40.1
+114	5	14.5	43.5
+115	5	15.65	30.15
+116	6	NULL	10
+117	6	10	11.1
+118	6	1.1	11.1
+119	6	NULL	1.0999999999999996
+120	6	NULL	NULL
+121	6	NULL	2.1999999999999997
+122	6	2.2	22.3
+123	6	20.1	11.9
+124	6	-10.4	9.7
+125	6	NULL	-10.400000000000002
+126	6	NULL	NULL
+127	6	NULL	NULL
 drop table t1;

mysql-test/t/win_sum.test

@@ -21,12 +21,27 @@ insert into t1 values
 (112 , 5, 1, 12.25),
 (113 , 5, NULL, 13.35),
 (114 , 5, NULL, 14.50),
-(115 , 5, NULL, 15.65);
+(115 , 5, NULL, 15.65),
+(116 , 6, 1, NULL),
+(117 , 6, 1, 10),
+(118 , 6, 1, 1.1),
+(119 , 6, 1, NULL),
+(120 , 6, 1, NULL),
+(121 , 6, 1, NULL),
+(122 , 6, 1, 2.2),
+(123 , 6, 1, 20.1),
+(124 , 6, 1, -10.4),
+(125 , 6, 1, NULL),
+(126 , 6, 1, NULL),
+(127 , 6, 1, NULL);
+
 
 --sorted_result
-select 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)
+select pk, a, b, sum(b) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
+from t1;
 
+--sorted_result
+select pk, a, c, sum(c) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING)
 from t1;
 
 drop table t1;

sql/item_windowfunc.h

@@ -490,7 +490,7 @@ public:
     else if (read_value_from_result_field)
     {
       res= result_field->val_real();
-      null_value= false;
+      null_value= result_field->is_null();
     }
     else
     {
@@ -563,7 +563,7 @@ public:
     }
     else
     {
-      res= result_field->val_decimal(dec);
+      res= window_func->val_decimal(dec);
       null_value= window_func->null_value;
     }
     return res;