This bug manifested itself when the server processed a query containing
a derived table over union whose ORDER BY clause included a subquery
with unresolvable column reference. For such a query the server crashed
when trying to resolve column references in the ORDER BY clause used by
union.
For any union with ORDER BY clause an extra SELECT_LEX structure is created
and it is attached to SELECT_LEX_UNIT structure of the union via the field
fake_select_lex. The outer context for fake_select_lex must be the same as
for other selects of the union. If the union is used in the FROM list of
a derived table then the outer context for fake_select_lex must be set to
NULL in line with other selects of the union. It was not done and it
caused a crash when searching for possible resolution of an unresolvable
column reference occurred in a subquery used in the ORDER BY clause.

Approved by Oleksandr Byelkin <sanja@mariadb.com>