pgsql: Fix snapshot handling in logicalmsg_decode

Fix snapshot handling in logicalmsg_decode

Whe decoding a transactional logical message, logicalmsg_decode called
SnapBuildGetOrBuildSnapshot. But we may not have a consistent snapshot
yet at that point. We don't actually need the snapshot in this case
(during replay we'll have the snapshot from the transaction), so in
practice this is harmless. But in assert-enabled build this crashes.

Fixed by requesting the snapshot only in non-transactional case, where
we are guaranteed to have SNAPBUILD_CONSISTENT.

Backpatch to 11. The issue exists since 9.6.

Backpatch-through: 11
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/84d60912-6eab-9b84-5de3-41765a5449e8@enterprisedb.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/949ac32e12674d9c0bcd3d95ea5e56338a567a18

Modified Files
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src/backend/replication/logical/decode.c | 14 ++++++++++++--
src/backend/replication/logical/reorderbuffer.c | 10 ++++++++++
2 files changed, 22 insertions(+), 2 deletions(-)