Problem: Some queries with subqueries and a HAVING clause that
consists only of a column not in the select or grouping lists causes
the server to crash.

During parsing, an Item_ref is constructed for the HAVING column. The
name of the column is resolved when JOIN::prepare calls fix_fields()
on its having clause. Since the column is not mentioned in the select
or grouping lists, a ref pointer is not found and a new Item_field is
created instead. The Item_ref is replaced by the Item_field in the
tree of HAVING clauses. Since the tree consists only of this item, the
pointer that is updated is JOIN::having. However,
st_select_lex::having still points to the Item_ref as the root of the
tree of HAVING clauses.

The bug is triggered when doing filesort for create_sort_index(). When
find_all_keys() calls select->cond->walk() it eventually reaches
Item_subselect::walk() where it continues to walk the having clauses
from lex->having. This means that it finds the Item_ref instead of the
new Item_field, and Item_ref::walk() tries to dereference the ref
pointer, which is still null.

The crash is reproducible only in 5.5, but the problem lies latent in
5.1 and trunk as well.

Fix: After calling fix_fields on the having clause in JOIN::prepare(),
set select_lex::having to point to the same item as JOIN::having.

This patch also fixes a bug in 5.1 and 5.5 that is triggered if the
query is executed as a prepared statement. The Item_field is created
in the runtime arena when the query is prepared, and the pointer to
the item is saved by st_select_lex::fix_prepare_information() and
brought back as a dangling pointer when the query is executed, after
the runtime arena has been reclaimed.

Fix: Backport fix from trunk that switches to the permanent arena
before calling Item_ref::fix_fields() in JOIN::prepare().