MDEV-15746 ASAN heap-use-after-free in Item_change_list::rollback_ite…

…m_tree_changes on ALTER executed as PS

10.3+ fix

On ALTER TABLE, if a non-changed column default
might need a charset conversion, it must
be a blob. Because blob's defaults ar stored
as expressions, and for any other type
a basic_const_item() will be in the record,
so it'll have correct charset and won't need
converting. For the same reason it makes no
sense to convert blob defaults (and it's
unsafe, see MDEV-15746).

test case is already in main/ps.test

sql/sql_table.cc

@@ -3191,8 +3191,10 @@ bool Column_definition::prepare_stage1_string(THD *thd,
     Convert the default value from client character
     set into the column character set if necessary.
     We can only do this for constants as we have not yet run fix_fields.
+    But not for blobs, as they will be stored as SQL expressions, not
+    written down into the record image.
   */
-  if (default_value &&
+  if (!(flags & BLOB_FLAG) && default_value &&
       default_value->expr->basic_const_item() &&
       charset != default_value->expr->collation.collation)
   {