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(at)gmail(dot)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(at)entry=6, no_tid=no_tid(at)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(at)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).