MDEV-5717: Server crash with insert statement containing DEFAULT into view

Item_default_value::arg can be NULL so walk() should take it into consideration.

mysql-test/r/view.result

@@ -4790,6 +4790,22 @@ v1_field1
 deallocate prepare my_stmt;
 DROP VIEW v1,v2;
 DROP TABLE t1,t2,t3,t4;
+#
+#MDEV-5717: Server crash with insert statement containing DEFAULT into
+#view
+#
+CREATE TABLE t1 (
+`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
+`test` tinyint(3) unsigned NOT NULL DEFAULT '0',
+PRIMARY KEY (`id`)
+);
+CREATE VIEW v1 AS (select  t1.id AS id,  t1.test AS test from t1);
+INSERT INTO v1 SET test = DEFAULT;
+select * from v1;
+id	test
+1	0
+drop view v1;
+drop table t1;
 # -----------------------------------------------------------------
 # -- End of 5.3 tests.
 # -----------------------------------------------------------------

mysql-test/t/view.test

@@ -4720,6 +4720,25 @@ deallocate prepare my_stmt;
 DROP VIEW v1,v2;
 DROP TABLE t1,t2,t3,t4;
 
+--echo #
+--echo #MDEV-5717: Server crash with insert statement containing DEFAULT into
+--echo #view
+--echo #
+CREATE TABLE t1 (
+  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
+  `test` tinyint(3) unsigned NOT NULL DEFAULT '0',
+  PRIMARY KEY (`id`)
+);
+
+CREATE VIEW v1 AS (select  t1.id AS id,  t1.test AS test from t1);
+
+INSERT INTO v1 SET test = DEFAULT;
+
+select * from v1;
+
+drop view v1;
+drop table t1;
+
 --echo # -----------------------------------------------------------------
 --echo # -- End of 5.3 tests.
 --echo # -----------------------------------------------------------------

sql/item.h

@@ -3551,7 +3551,7 @@ class Item_default_value : public Item_field
 
   bool walk(Item_processor processor, bool walk_subquery, uchar *args)
   {
-    return arg->walk(processor, walk_subquery, args) ||
+    return (arg && arg->walk(processor, walk_subquery, args)) ||
       (this->*processor)(args);
   }