pgsql: Make GetSnapshotData() more resilient on out-of-memory errors

Make GetSnapshotData() more resilient on out-of-memory errors

If the allocation of Snapshot->subxip fails, a follow-up call of
GetSnapshotData() would see a partially-initialized snapshot, causing a
NULL dereference on reentry when using "subxip" because only "xip" would
be allocated. In the event of an out-of-memory error when allocating
"subxip", "xip" is now reset before throwing an ERROR, so as Snapshots
can be allocated and handled gracefully on retry.

Note: This has been originally applied in v19~ as of 29fb598b9cad, and
was initially not backpatched. This issue has been seen in the field on
older stable branches, hence the backpatch now.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Author: Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>
Discussion: https://postgr.es/m/e77acaac-a1b3-40b3-99ee-5769b4e453e4@gmail.com
Discussion: https://postgr.es/m/CAJ7c6TO+7rrN5H_Odd8PLdmrEa4MgeoJ2AqnTydOcy=AyZvDsQ@mail.gmail.com
Backpatch-through: 15

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/94641eda4c6293fa43b44ff8c7433b510f4bcfa5

Modified Files
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src/backend/storage/ipc/procarray.c | 8 ++++++++
1 file changed, 8 insertions(+)