BUG #19494: Error on transaction commit inside pipeline triggers psql's Assert

The following bug has been logged on the website:

Bug reference:      19494
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 18.4
Operating system:   Ubuntu 24.04
Description:        

The following psql script:
CREATE TABLE t(a INTEGER PRIMARY KEY DEFERRABLE INITIALLY DEFERRED);
\startpipeline
INSERT INTO t VALUES ($1), ($1) RETURNING * \bind 1 \sendpipeline
\endpipeline

results in:
 a
---
 1
 1
(2 rows)

ERROR:  duplicate key value violates unique constraint "t_pkey"
DETAIL:  Key (a)=(1) already exists.
psql: common.c:1503: discardAbortedPipelineResults: Assertion
`pset.available_results > 0' failed.

Program terminated with signal SIGABRT, Aborted.
(gdb) bt
#0  __pthread_kill_implementation (threadid=281473811984096,
signo=signo@entry=6, no_tid=no_tid@entry=0)
    at ./nptl/pthread_kill.c:44
#1  0x0000ffffba60b718 [PAC] in __pthread_kill_internal (threadid=<optimized
out>, signo=6) at ./nptl/pthread_kill.c:89
#2  0x0000ffffba5b757c in __GI_raise (sig=sig@entry=6) at
../sysdeps/posix/raise.c:26
#3  0x0000ffffba5a1d48 [PAC] in __GI_abort () at ./stdlib/abort.c:77
#4  0x0000ffffba5b05e0 [PAC] in __assert_fail_base (fmt=<optimized out>,
assertion=<optimized out>,
    file=0xaaaac5a317a8 "common.c", line=1503, function=<optimized out>) at
./assert/assert.c:118
#5  0x0000aaaac59d3ffc [PAC] in discardAbortedPipelineResults () at
common.c:1503
#6  0x0000aaaac59d4b28 in ExecQueryAndProcessResults (query=0xaaaae5929250
"INSERT INTO t VALUES ($1), ($1) RETURNING * ",
    elapsed_msec=0xffffe4d657b0, svpt_gone_p=0xffffe4d657af, is_watch=false,
min_rows=0, opt=0x0, printQueryFout=0x0)
    at common.c:1830
#7  0x0000aaaac59d37b0 in SendQuery (query=0xaaaae5929250 "INSERT INTO t
VALUES ($1), ($1) RETURNING * ") at common.c:1214
#8  0x0000aaaac59ee4f8 in MainLoop (source=0xffffba730870 <_IO_2_1_stdin_>)
at mainloop.c:515
#9  0x0000aaaac59cd488 in process_file (filename=0x0,
use_relative_path=false) at command.c:4977
#10 0x0000aaaac59fbbcc in main (argc=10, argv=0xffffe4d65f68) at
startup.c:424

Reproduced starting from 41625ab8e (with s/\sendpipeline/\g/ before
17caf6644).