Handle null values correctly in SP local variables.

mysql-test/r/sp.result:
  Test for handling of null values.
mysql-test/t/sp.test:
  Test for handling of null values.
sql/item.h:
  Update null_value correctly in Item_splocal.

mysql-test/r/sp.result

@@ -85,11 +85,23 @@ declare lf double;
 declare ls varchar(32);
 set ld = null, li = null, lf = null, ls = null;
 insert into t3 values (ld, li, lf, ls);
+insert into t3 (i, f, s) values ((ld is null), 1,    "ld is null"),
+((li is null), 1,    "li is null"),
+((li = 0),     null, "li = 0"),
+((lf is null), 1,    "lf is null"),
+((lf = 0),     null, "lf = 0"),
+((ls is null), 1,    "ls is null");
 end;
 call nullset();
 select * from t3;
 d	i	f	s
 NULL	NULL	NULL	NULL
+NULL	1	1	ld is null
+NULL	1	1	li is null
+NULL	NULL	NULL	li = 0
+NULL	1	1	lf is null
+NULL	NULL	NULL	lf = 0
+NULL	1	1	ls is null
 drop table t3;
 drop procedure nullset;
 create procedure mixset(x char(16), y int)

mysql-test/t/sp.test

@@ -124,6 +124,13 @@ begin
 
   set ld = null, li = null, lf = null, ls = null;
   insert into t3 values (ld, li, lf, ls);
+
+  insert into t3 (i, f, s) values ((ld is null), 1,    "ld is null"),
+                                  ((li is null), 1,    "li is null"),
+				  ((li = 0),     null, "li = 0"),
+				  ((lf is null), 1,    "lf is null"),
+				  ((lf = 0),     null, "lf = 0"),
+				  ((ls is null), 1,    "ls is null");
 end|
 
 call nullset()|

sql/item.h

@@ -256,17 +256,34 @@ class Item_splocal : public Item
 
   inline double val()
   {
-    return this_item()->val();
+    Item *it= this_item();
+    double ret= it->val();
+    Item::null_value= it->null_value;
+    return ret;
   }
 
   inline longlong val_int()
   {
-    return this_item()->val_int();
+    Item *it= this_item();
+    longlong ret= it->val_int();
+    Item::null_value= it->null_value;
+    return ret;
   }
 
   inline String *val_str(String *sp)
   {
-    return this_item()->val_str(sp);
+    Item *it= this_item();
+    String *ret= it->val_str(sp);
+    Item::null_value= it->null_value;
+    return ret;
+  }
+
+  inline bool is_null()
+  {
+    Item *it= this_item();
+    bool ret= it->is_null();
+    Item::null_value= it->null_value;
+    return ret;
   }
 
   inline void make_field(Send_field *field)