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
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b44ae4893db20233b470e41916b67799a1738bf4

Modified Files
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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(-)