Bug#31095: Unexpected NULL constant caused server crash.

The Item_func_rollup_const class is used for wrapping constants to avoid
wrong result for ROLLUP queries with DISTINCT and a constant in the select
list. This class is also used to wrap up a NULL constant but its null_value
wasn't set accordingly. This led to a server crash.

Now the null_value of an object of the Item_func_rollup_const class is set
by its fix_length_and_dec member function.

mysql-test/r/olap.result

@@ -715,3 +715,14 @@ a	SUM(a)
 4	4
 NULL	14
 DROP TABLE t1;
+#
+# Bug#31095: Unexpected NULL constant caused server crash.
+#
+create table t1(a int);
+insert into t1 values (1),(2),(3);
+select count(a) from t1 group by null with rollup;
+count(a)
+3
+3
+drop table t1;
+##############################################################

mysql-test/t/olap.test

@@ -358,3 +358,12 @@ SELECT * FROM (SELECT a, SUM(a) FROM t1 GROUP BY a WITH ROLLUP) as t;
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug#31095: Unexpected NULL constant caused server crash.
+--echo #
+create table t1(a int);
+insert into t1 values (1),(2),(3);
+select count(a) from t1 group by null with rollup;
+drop table t1;
+--echo ##############################################################
+

sql/item_func.h

@@ -749,6 +749,8 @@ public:
     collation= args[0]->collation;
     max_length= args[0]->max_length;
     decimals=args[0]->decimals; 
+    /* The item could be a NULL constant. */
+    null_value= args[0]->null_value;
   }
 };