pgsql: Fix memory leak in ARRAY(SELECT ...) subqueries.

Fix memory leak in ARRAY(SELECT ...) subqueries.

Repeated execution of an uncorrelated ARRAY_SUBLINK sub-select (which
I think can only happen if the sub-select is embedded in a larger,
correlated subquery) would leak memory for the duration of the query,
due to not reclaiming the array generated in the previous execution.
Per bug #6698 from Armando Miraglia. Diagnosis and fix idea by Heikki,
patch itself by me.

This has been like this all along, so back-patch to all supported versions.

Branch
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REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/248f1ac813e7a5fd83816b1d85a3d4a14e7574a2

Modified Files
--------------
src/backend/executor/nodeSubplan.c | 18 +++++++++++++-----
src/include/nodes/execnodes.h | 1 +
2 files changed, 14 insertions(+), 5 deletions(-)