Fix nbtree skip array parallel alloc accounting.

Fix nbtree skip array parallel alloc accounting.

btestimateparallelscan neglected to add btps_arrElems[] space overhead
for skip array scan keys that were later output by nbtree preprocessing.
Skip arrays don't actually need to use this space, but a scan with a
subsequent SAOP array will need to subscript btps_arrElems[] using a
simple so->arrayKeys[]-wise offset.  so->arrayKeys[] has entries for
both kinds of arrays.

As a result of this oversight, it was possible for an index scan with a
skip array and a lower-order SAOP array to write past the allocated
shared memory boundary when storing the SAOP array's cur_elem.  In
practice the problem seems to be limited to scans with many skipped
index columns, since our general approach to estimating the amount of
shared memory that will be required is fairly conservative.

To fix, have btestimateparallelscan request an extra sizeof(int) space
for key columns that might require a skip array later on.

Oversight in commit 92fe23d9, which added the nbtree skip scan
optimization.

Author: Siddharth Kothari <sidkot@google.com>
Discussion: https://postgr.es/m/CAGCUe0Lwk3C0qdkBa+OLpYc7yXwW=pbaz8Sju4xMXEQAmyp+5g@mail.gmail.com
Backpatch-through: 18