BUG #16393: PANIC: cannot abort transaction, it was already committed

The following bug has been logged on the website:

Bug reference:      16393
Logged by:          Andreas Seltenreich
Email address:      andreas.seltenreich@credativ.de
PostgreSQL version: 11.5
Operating system:   AIX
Description:

One of our customers recently experienced the following PANIC with
PostgreSQL 11.5 on AIX:

PANIC:  cannot abort transaction 1234567890, it was already committed
CONTEXT:  PL/pgSQL function foo.bar(varchar[],…,timestamptz[]) line 5 during
exception cleanup
STATEMENT:  PREPARE q1 (varchar,[...],timestamptz) AS select
foo.bar(ARRAY[$1, …], …, $101], ARRAY[$102, $103])

There is a striking similarity to BUG #15727 from 2019 where this
PANIC also occured during error handling.  Archive link to the old
bug: https://postgr.es/m/15727-0be246e7d852d229@postgresql.org

IIUC backporting was considered but deemed unreasonable here:
https://postgr.es/m/20190407203315.6v3iktksr5u3bksw@alap3.anarazel.de

Backtrace below.

regards,
Andreas

(dbx) where
abort()
pq_getmsgtext()
errposition()
IPRA.$AtCleanup_Memory()
StartTransaction()
DefineSavepoint()
IPRA.$exec_stmt_block()
pl_exec.IPRA.$exec_stmt_raise.exec_stmt()
pl_exec.IPRA.$exec_stmt_raise.IPRA.$exec_stmts()
IPRA.$exec_stmt_block()
plpgsql_exec_function()
plpgsql_call_handler()
ExecInterpExpr()
ExecEvalConstraintNotNull@AF173_67()
ExecInitProjectSet()
MultiExecProcNode()
IPRA.$InitPlan()
IPRA.$ExecBuildAggTransCall()
PortalRunFetch()