pgsql: Fix assertion after aborting internal subtransaction at transact

Fix assertion after aborting internal subtransaction at transaction end

Previously, aborting an internal subtransaction during COMMIT or
PREPARE TRANSACTION could cause the following assertion failure.
This could happen, for example, when a deferred constraint trigger fired
at COMMIT and its PL/pgSQL exception block caught an error raised
while executing the trigger function.

TRAP: failed Assert("s->blockState == TBLOCK_SUBINPROGRESS || s->blockState
== TBLOCK_INPROGRESS || s->blockState == TBLOCK_IMPLICIT_INPROGRESS ||
s->blockState == TBLOCK_PARALLEL_INPROGRESS || s->blockState ==
TBLOCK_STARTED"), File: "xact.c", Line: 4851, PID: 73455

An internal subtransaction should be able to be aborted while the parent
transaction is in the COMMIT or PREPARE TRANSACTION phase. However,
RollbackAndReleaseCurrentSubTransaction()'s assertion check previously
did not allow TBLOCK_END and TBLOCK_PREPARE as parent transaction
states, causing the assertion failure.

This commit fixes the assertion check by allowing those two parent
transaction states.

Backpatch to all supported versions.

Reported-by: Fabrízio de Royes Mello <fabrizio(at)planetscale(dot)com>
Author: Patrick Reynolds <piki(at)planetscale(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Fabrízio de Royes Mello <fabrizio(at)planetscale(dot)com>
Discussion: https://postgr.es/m/CABo-N97AeMbWuYTWg-3%3D2DkTR3EkvS%2BFt%3DyEaWB181STsR1mBg%40mail.gmail.com
Backpatch-through: 14

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e371cb7b89dddd9c1d3d89f1e5c64dabc05474aa

Modified Files
--------------
src/backend/access/transam/xact.c | 2 ++
src/test/regress/expected/triggers.out | 28 ++++++++++++++++++++++++++++
src/test/regress/sql/triggers.sql | 25 +++++++++++++++++++++++++
3 files changed, 55 insertions(+)