Fix for bug #32558: group by null-returning expression with rollup causes crash

Problem: setting Item_func_rollup_const::null_value property to argument's null_value
before (without) the argument evaluation may result in a crash due to wrong null_value.

Fix: use is_null() to set Item_func_rollup_const::null_value instead as it evaluates
the argument if necessary and returns a proper value.

mysql-test/r/olap.result

@@ -726,3 +726,11 @@ count(a)
 3
 drop table t1;
 ##############################################################
+CREATE TABLE t1(a INT);
+INSERT INTO t1 VALUES(0);
+SELECT 1 FROM t1 GROUP BY (DATE(NULL)) WITH ROLLUP;
+1
+1
+1
+DROP TABLE t1;
+End of 5.0 tests

mysql-test/t/olap.test

@@ -367,3 +367,12 @@ select count(a) from t1 group by null with rollup;
 drop table t1;
 --echo ##############################################################
 
+#
+# Bug #32558: group by null-returning expression with rollup causes crash
+#
+CREATE TABLE t1(a INT);
+INSERT INTO t1 VALUES(0);
+SELECT 1 FROM t1 GROUP BY (DATE(NULL)) WITH ROLLUP;
+DROP TABLE t1;
+
+--echo End of 5.0 tests

sql/item_func.h

@@ -782,7 +782,7 @@ public:
     max_length= args[0]->max_length;
     decimals=args[0]->decimals; 
     /* The item could be a NULL constant. */
-    null_value= args[0]->null_value;
+    null_value= args[0]->is_null();
   }
 };