pgsql: MAXALIGN the target address where we store flattened value.

MAXALIGN the target address where we store flattened value.

The API (EOH_flatten_into) that flattens the expanded value representation
expects the target address to be maxaligned.  All it's usage adhere to that
principle except when serializing datums for parallel query.  Fix that
usage.

Diagnosed-by: Tom Lane
Author: Tom Lane and Amit Kapila
Backpatch-through: 9.6
Discussion: https://postgr.es/m/11629.1536550032@sss.pgh.pa.us

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/9718c93f532c3cfb6c39679452149dba711d12af

Modified Files
--------------
src/backend/utils/adt/datum.c                 | 13 ++++++++++++-
src/test/regress/expected/select_parallel.out | 27 +++++++++++++++++++++++++++
src/test/regress/sql/select_parallel.sql      | 16 ++++++++++++++++
3 files changed, 55 insertions(+), 1 deletion(-)