Fix rare sharedtuplestore.c corruption.
If the final chunk of an oversized tuple being written out to disk was
exactly 32760 bytes, it would be corrupted due to a fencepost bug.
Bug #17619. Back-patch to 11 where the code arrived.
While testing that (see test module in archives), I (tmunro) noticed
that the per-participant page counter was not initialized to zero as it
should have been; that wasn't a live bug when it was written since DSM
memory was originally always zeroed, but since 14
min_dynamic_shared_memory might be configured and it supplies non-zeroed
memory, so that is also fixed here.
Author: Dmitry Astapov <dastapov@gmail.com>
Discussion: https://postgr.es/m/17619-
0de62ceda812b8b5%40postgresql.org