MDEV-15318 CREATE .. SELECT VALUES produces invalid table structure

When Item_insert_value needs a dummy field,
use zero-length Field_string, not Field_null.
The latter isn't compatible with CREATE ... SELECT.

mysql-test/r/insert_select.result

@@ -853,3 +853,12 @@ INSERT IGNORE INTO t1 SELECT t1.a FROM t1,t1 t2,t1 t3,t1 t4,t1 t5,t1 t6,t1 t7;
 SET GLOBAL myisam_data_pointer_size = @old_myisam_data_pointer_size;
 DROP TABLE t1;
 End of 5.1 tests
+create table t1 (i int);
+create table t2 as select values(i) as a from t1;
+show create table t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `a` binary(0) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+drop table t1, t2;
+End of 5.5 tests

mysql-test/t/insert_select.test

@@ -423,3 +423,13 @@ SET GLOBAL myisam_data_pointer_size = @old_myisam_data_pointer_size;
 DROP TABLE t1;
 
 --echo End of 5.1 tests
+
+#
+# MDEV-15318 CREATE .. SELECT VALUES produces invalid table structure
+#
+create table t1 (i int);
+create table t2 as select values(i) as a from t1;
+show create table t2;
+drop table t1, t2;
+
+--echo End of 5.5 tests

sql/item.cc

@@ -8541,10 +8541,10 @@ bool Item_insert_value::fix_fields(THD *thd, Item **items)
   }
   else
   {
-    Field *tmp_field= field_arg->field;
-    /* charset doesn't matter here, it's to avoid sigsegv only */
-    tmp_field= new Field_null(0, 0, Field::NONE, field_arg->field->field_name,
-                          &my_charset_bin);
+    static uchar null_bit=1;
+    /* charset doesn't matter here */
+    Field *tmp_field= new Field_string(0, 0, &null_bit, 1, Field::NONE,
+                                field_arg->field->field_name, &my_charset_bin);
     if (tmp_field)
     {
       tmp_field->init(field_arg->field->table);