BUG#31450: Query causes error 1048

- Let Item::save_in_field() call set_field_to_null_with_conversions() 
  for decimal type, like this is done for the other item result types.

mysql-test/r/type_decimal.result

@@ -805,3 +805,14 @@ SELECT 1 % .12345678912345678912345678912345678912345678912345678912345678912345
 SELECT MOD(1, .123456789123456789123456789123456789123456789123456789123456789123456789123456789) AS 'MOD()';
 MOD()
 0.012345687012345687012345687012345687012345687012345687012345687012345687000000000
+create table t1 (
+ua_id decimal(22,0) not null,
+ua_invited_by_id decimal(22,0) default NULL,
+primary key(ua_id)
+);
+insert into t1 values (123, NULL), (456, NULL);
+this must not produce error 1048: 
+select * from t1 where ua_invited_by_id not in (select ua_id from t1);
+ua_id	ua_invited_by_id
+drop table t1;
+End of 5.0 tests

mysql-test/t/type_decimal.test

@@ -416,3 +416,22 @@ DROP TABLE t1;
 
 SELECT 1 % .123456789123456789123456789123456789123456789123456789123456789123456789123456789 AS '%';
 SELECT MOD(1, .123456789123456789123456789123456789123456789123456789123456789123456789123456789) AS 'MOD()';
+
+
+#
+# BUG#31450 "Query causes error 1048"
+#
+create table t1 (
+  ua_id decimal(22,0) not null,
+  ua_invited_by_id decimal(22,0) default NULL,
+  primary key(ua_id)
+);
+insert into t1 values (123, NULL), (456, NULL);
+
+--echo this must not produce error 1048: 
+select * from t1 where ua_invited_by_id not in (select ua_id from t1);
+
+drop table t1;
+
+--echo End of 5.0 tests
+

sql/item.cc

@@ -4548,7 +4548,7 @@ int Item::save_in_field(Field *field, bool no_conversions)
     my_decimal decimal_value;
     my_decimal *value= val_decimal(&decimal_value);
     if (null_value)
-      return set_field_to_null(field);
+      return set_field_to_null_with_conversions(field, no_conversions);
     field->set_notnull();
     error=field->store_decimal(value);
   }