Bug#31160: MAKETIME() crashes server when returning NULL in ORDER BY using filesort

Even though it returns NULL, the MAKETIME function did not have this property set,
causing a failed assertion (designed to catch exactly this).
Fixed by setting the nullability property of MAKETIME().

mysql-test/r/func_sapdb.result

@@ -196,7 +196,7 @@ f2	datetime	YES		NULL
 f3	time	YES		NULL	
 f4	time	YES		NULL	
 f5	time	YES		NULL	
-f6	time	NO		00:00:00	
+f6	time	YES		NULL	
 f7	datetime	YES		NULL	
 f8	date	YES		NULL	
 f9	time	YES		NULL	

mysql-test/r/func_time.result

@@ -1027,6 +1027,15 @@ fmtddate	field2
 Sep-4 12:00AM	abcd
 DROP TABLE testBug8868;
 SET NAMES DEFAULT;
+CREATE TABLE t1 (
+a TIMESTAMP
+);
+INSERT INTO t1 VALUES (now()), (now());
+SELECT 1 FROM t1 ORDER BY MAKETIME(1, 1, a);
+1
+1
+1
+DROP TABLE t1;
 (select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%H') As H)
 union
 (select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 DAY)),'%H') As H);

mysql-test/t/func_time.test

@@ -545,6 +545,16 @@ DROP TABLE testBug8868;
 
 SET NAMES DEFAULT;
 
+#
+# Bug #31160: MAKETIME() crashes server when returning NULL in ORDER BY using 
+# filesort
+#
+CREATE TABLE t1 (
+  a TIMESTAMP
+);
+INSERT INTO t1 VALUES (now()), (now());
+SELECT 1 FROM t1 ORDER BY MAKETIME(1, 1, a);
+DROP TABLE t1;
 
 #
 # Bug #19844 time_format in Union truncates values

sql/item_timefunc.h

@@ -962,7 +962,10 @@ class Item_func_maketime :public Item_str_timefunc
 {
 public:
   Item_func_maketime(Item *a, Item *b, Item *c)
-    :Item_str_timefunc(a, b ,c) {}
+    :Item_str_timefunc(a, b, c) 
+  {
+    maybe_null= TRUE;
+  }
   String *val_str(String *str);
   const char *func_name() const { return "maketime"; }
 };