pgsql: Fix intra-query memory leakage in nodeProjectSet.c.

Fix intra-query memory leakage in nodeProjectSet.c.

Both ExecMakeFunctionResultSet() and evaluation of simple expressions
need to be done in the per-tuple memory context, not per-query, else
we leak data until end of query. This is a consideration that was
missed while refactoring code in the ProjectSet patch (note that in
pre-v10, ExecMakeFunctionResult is called in the per-tuple context).

Per bug #14843 from Ben M. Diagnosed independently by Andres and myself.

Discussion: https://postgr.es/m/20171005230321.28561.15927@wrigleys.postgresql.org

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0c25e9652461c08b5caef259a6af27a38707e07a

Modified Files
--------------
src/backend/executor/execSRF.c | 2 ++
src/backend/executor/nodeProjectSet.c | 6 ++++++
2 files changed, 8 insertions(+)