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