pgsql: Fix trigger WHEN conditions when both BEFORE and AFTER triggers

Fix trigger WHEN conditions when both BEFORE and AFTER triggers exist.

Due to tuple-slot mismanagement, evaluation of WHEN conditions for AFTER
ROW UPDATE triggers could crash if there had been a BEFORE ROW trigger
fired for the same update. Fix by not trying to overload the use of
estate->es_trig_tuple_slot. Per report from Yoran Heling.

Back-patch to 9.0, when trigger WHEN conditions were introduced.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/38c9eb8fee298a9242bb2049c4fdf94407187d0c

Modified Files
--------------
src/backend/commands/trigger.c | 6 +++---
src/backend/executor/execMain.c | 1 +
src/backend/executor/execUtils.c | 1 +
src/include/nodes/execnodes.h | 8 +++++++-
src/test/regress/expected/triggers.out | 29 +++++++++++++++++++++++++++++
src/test/regress/sql/triggers.sql | 26 ++++++++++++++++++++++++++
6 files changed, 67 insertions(+), 4 deletions(-)