Server crash in Item_field::fix_outer_field with nested subquery using GROUP BY and HAVING.

I can reproduce a debug-build server crash caused by nested subqueries with outer
references.The crash happens in Item_field::fix_outer_field at sql/item.cc:6152, with the following
assertion:
DBUG_ASSERT(*ref && (*ref)->fixed());
The issue reproduces with a very small query and seems related to outer-reference resolution during JOIN::prepare() for nested subqueries.

Version:

12.0.1-MariaDB-debug
source revision: ecb7c9b692811f96cfa54add61012a701c47523d

Steps to Reproduce:
```
DROP TABLE IF EXISTS t;

CREATE TABLE t(c INT);

SELECT
(SELECT 0 GROUP BY c HAVING (SELECT c))
FROM t
GROUP BY c;

DROP TABLE t;
```

backtrace:
```
sql/item.cc:6152(Item_field::fix_outer_field)
sql/item.cc:6450(Item_field::fix_fields)
sql/item.h:1113(Item::fix_fields_if_needed)
sql/item.h:1122(Item::fix_fields_if_needed_for_scalar)
sql/sql_base.cc:8117(setup_fields)
sql/sql_select.cc:1572(JOIN::prepare)
sql/item_subselect.cc:3981(subselect_single_select_engine::prepare)
sql/item_subselect.cc:294(Item_subselect::fix_fields)
sql/sql_select.cc:5350(mysql_select)
sql/sql_parse.cc:6191(execute_sqlcom_select)
```