pgsql: Ensure proper alignment of tuples in HashMemoryChunkDatabuffers

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
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master

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

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