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_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ca5ca25d08710d7f5aaf0e390aae284bc594a6da

Modified Files
--------------
src/backend/utils/adt/datum.c                 | 13 +++++++++++-
src/test/regress/expected/select_parallel.out | 29 ++++++++++++++++++++++++++-
src/test/regress/sql/select_parallel.sql      | 18 ++++++++++++++++-
3 files changed, 57 insertions(+), 3 deletions(-)