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
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/dca44d07c585637d8245a46a29be732241fa40bf

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(-)