fix for bug #23938: ISNULL on DATE AND CAST AS DATE returns false for null values

Set null_value in case of wrong data.

mysql-test/r/cast.result

@@ -278,3 +278,7 @@ double_val	cast_val
 -1e+30	-9223372036854775808
 1e+30	9223372036854775807
 DROP TABLE t1;
+select isnull(date(NULL)), isnull(cast(NULL as DATE));
+isnull(date(NULL))	isnull(cast(NULL as DATE))
+1	1
+End of 4.1 tests

mysql-test/t/cast.test

@@ -167,4 +167,10 @@ INSERT INTO t1 SET f1 = +1.0e+30 ;
 SELECT f1 AS double_val, CAST(f1 AS SIGNED INT) AS cast_val FROM t1;
 DROP TABLE t1;					   
 
-# End of 4.1 tests
+#
+# Bug #23938: cast(NULL as DATE)
+#
+
+select isnull(date(NULL)), isnull(cast(NULL as DATE));
+
+--echo End of 4.1 tests

sql/item_timefunc.cc

@@ -2507,7 +2507,10 @@ longlong Item_date_typecast::val_int()
   DBUG_ASSERT(fixed == 1);
   TIME ltime;
   if (args[0]->get_date(&ltime, TIME_FUZZY_DATE))
+  {
+    null_value= 1;
     return 0;
+  }
   return (longlong) (ltime.year * 10000L + ltime.month * 100 + ltime.day);
 }