pgsql: Ensure proper alignment of tuples in HashMemoryChunkData buffers

Ensure proper alignment of tuples in HashMemoryChunkData buffers.

The previous coding relied (without any documentation) on the data[]
member of HashMemoryChunkData being at a MAXALIGN'ed offset. If it
was not, the tuples would not be maxaligned either, leading to failures
on alignment-picky machines. While there seems to be no live bug on any
platform we support, this is clearly pretty fragile: any addition to or
rearrangement of the fields in HashMemoryChunkData could break it.
Let's remove the hazard by getting rid of the data[] member and instead
using pointer arithmetic with an explicitly maxalign'ed offset.

Discussion: https://postgr.es/m/14483.1514938129@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/5dc692f78d3bee1e86d095a9e8d9242b44f78b01

Modified Files
--------------
src/backend/executor/nodeHash.c | 34 +++++++++++++++-------------------
src/include/executor/hashjoin.h | 12 +++++++++---
2 files changed, 24 insertions(+), 22 deletions(-)