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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master

Details
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http://git.postgresql.org/pg/commitdiff/d14241c2cf72966c09c1acece5cb44b5c84204b4

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