Fix for bug #32557: order by updatexml causes assertion in filesort

Problem: even if an Item_xml_str_func successor returns NULL, it doesn't have 
a corresponding property (maybe_null) set, that leads to a failed assertion.

Fix: set nullability property of Item_xml_str_func.

mysql-test/r/xml.result

@@ -1022,4 +1022,11 @@ NULL	NULL	NULL
 select updatexml(NULL, NULL, NULL);
 updatexml(NULL, NULL, NULL)
 NULL
+CREATE TABLE t1(a INT NOT NULL);
+INSERT INTO t1 VALUES (0), (0);
+SELECT 1 FROM t1 ORDER BY(UPDATEXML(a, '1', '1'));
+1
+1
+1
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/xml.test

@@ -543,4 +543,12 @@ select updatexml(NULL, NULL, 1), updatexml(1, NULL, NULL),
        updatexml(NULL, 1, NULL);
 select updatexml(NULL, NULL, NULL);
 
+#
+# Bug #32557: order by updatexml causes assertion in filesort
+#
+CREATE TABLE t1(a INT NOT NULL);
+INSERT INTO t1 VALUES (0), (0);
+SELECT 1 FROM t1 ORDER BY(UPDATEXML(a, '1', '1'));
+DROP TABLE t1;
+
 --echo End of 5.1 tests

sql/item_xmlfunc.h

@@ -28,8 +28,16 @@ class Item_xml_str_func: public Item_str_func
   String tmp_value, pxml;
   Item *nodeset_func;
 public:
-  Item_xml_str_func(Item *a, Item *b): Item_str_func(a,b) {}
-  Item_xml_str_func(Item *a, Item *b, Item *c): Item_str_func(a,b,c) {}
+  Item_xml_str_func(Item *a, Item *b): 
+    Item_str_func(a,b) 
+  {
+    maybe_null= TRUE;
+  }
+  Item_xml_str_func(Item *a, Item *b, Item *c): 
+    Item_str_func(a,b,c) 
+  {
+    maybe_null= TRUE;
+  }
   void fix_length_and_dec();
   String *parse_xml(String *raw_xml, String *parsed_xml_buf);
 };