pgsql: Fix crash in assign_collations_walker for EXISTS with empty SELE

Fix crash in assign_collations_walker for EXISTS with empty SELECT list.

We (I think I, actually) forgot about this corner case while coding
collation resolution.  Per bug #8648 from Arjen Nienhuis.

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/6698782f19d938a475c46922ac90e366713fc273

Modified Files
--------------
src/backend/parser/parse_collate.c      |   10 ++++++++--
src/test/regress/expected/subselect.out |   10 ++++++++++
src/test/regress/sql/subselect.sql      |    6 ++++++
3 files changed, 24 insertions(+), 2 deletions(-)