BUG #16139: Assertion fails on INSERT into a postgres_fdw' table with two AFTER INSERT triggers

The following bug has been logged on the website:

Bug reference:      16139
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 12.1
Operating system:   Ubuntu 18.04
Description:

The following script:
CREATE EXTENSION postgres_fdw;
CREATE SERVER loopback FOREIGN DATA WRAPPER postgres_fdw OPTIONS (dbname
'postgres');
CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
CREATE TABLE loc2 (f1 int);
CREATE FOREIGN TABLE rem2 (f1 int) SERVER loopback OPTIONS(table_name
'loc2');
CREATE OR REPLACE FUNCTION trigger_data() RETURNS trigger
LANGUAGE plpgsql AS 'begin return NEW; end;';

CREATE TRIGGER trigger1 AFTER INSERT ON rem2 FOR EACH ROW EXECUTE PROCEDURE
trigger_data();
CREATE TRIGGER trigger2 AFTER INSERT ON rem2 FOR EACH ROW EXECUTE PROCEDURE
trigger_data();
INSERT INTO rem2 VALUES(1);

raises the assertion:
Core was generated by `postgres: law regression [local] INSERT
                        '.
Program terminated with signal SIGABRT, Aborted.
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
51      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007fd866e97801 in __GI_abort () at abort.c:79
#2  0x000055c2cfc2f022 in ExceptionalCondition (
    conditionName=conditionName@entry=0x55c2cfdaa228 "!(!(((slot)->tts_flags
& (1 << 1)) != 0))", 
    errorType=errorType@entry=0x55c2cfc86108 "FailedAssertion",
fileName=fileName@entry=0x55c2cfdc7e3b "execTuples.c", 
    lineNumber=lineNumber@entry=1620) at assert.c:54
#3  0x000055c2cf99380f in ExecFetchSlotHeapTuple
(slot=slot@entry=0x55c2d03b2bc8, materialize=materialize@entry=true, 
    shouldFree=shouldFree@entry=0x7ffd86cf260e) at execTuples.c:1620
#4  0x000055c2cf9609b7 in AfterTriggerExecute
(estate=estate@entry=0x55c2d039dd88, event=event@entry=0x55c2d03c0b7c, 
    relInfo=relInfo@entry=0x55c2d039e028,
trigdesc=trigdesc@entry=0x55c2d039e140, finfo=finfo@entry=0x55c2d039e300, 
    instr=instr@entry=0x0, per_tuple_context=0x55c2d03c2a50,
trig_tuple_slot1=0x55c2d03b2bc8, 
    trig_tuple_slot2=0x55c2d03b2c60) at trigger.c:4259
#5  0x000055c2cf960fa5 in afterTriggerInvokeEvents
(events=events@entry=0x55c2d034ebf8, firing_id=1, 
    estate=estate@entry=0x55c2d039dd88, delete_ok=delete_ok@entry=false) at
trigger.c:4540
#6  0x000055c2cf966876 in AfterTriggerEndQuery
(estate=estate@entry=0x55c2d039dd88) at trigger.c:4851
#7  0x000055c2cf986c53 in standard_ExecutorFinish (queryDesc=0x55c2d02edbc8)
at execMain.c:439
#8  0x000055c2cf986c74 in ExecutorFinish
(queryDesc=queryDesc@entry=0x55c2d02edbc8) at execMain.c:407
#9  0x000055c2cfb08771 in ProcessQuery (plan=plan@entry=0x55c2d03ac3d0, 
    sourceText=0x55c2d02ca3d8 "INSERT INTO rem2 VALUES(1);", params=0x0,
queryEnv=0x0, dest=dest@entry=0x55c2d03abe70, 
    completionTag=completionTag@entry=0x7ffd86cf29e0 "INSERT 0 1") at
pquery.c:203
#10 0x000055c2cfb088d2 in PortalRunMulti
(portal=portal@entry=0x55c2d0331498, isTopLevel=isTopLevel@entry=true, 
    setHoldSnapshot=setHoldSnapshot@entry=false,
dest=dest@entry=0x55c2d03abe70, altdest=altdest@entry=0x55c2d03abe70, 
    completionTag=completionTag@entry=0x7ffd86cf29e0 "INSERT 0 1") at
pquery.c:1283
#11 0x000055c2cfb0977d in PortalRun (portal=portal@entry=0x55c2d0331498,
count=count@entry=9223372036854775807, 
    isTopLevel=isTopLevel@entry=true, run_once=run_once@entry=true,
dest=dest@entry=0x55c2d03abe70, 
    altdest=altdest@entry=0x55c2d03abe70, completionTag=0x7ffd86cf29e0
"INSERT 0 1") at pquery.c:796
#12 0x000055c2cfb05a2d in exec_simple_query (
    query_string=query_string@entry=0x55c2d02ca3d8 "INSERT INTO rem2
VALUES(1);") at postgres.c:1215
#13 0x000055c2cfb079fd in PostgresMain (argc=<optimized out>,
argv=argv@entry=0x55c2d02f5598, dbname=<optimized out>, 
    username=<optimized out>) at postgres.c:4236
#14 0x000055c2cfa79e2d in BackendRun (port=port@entry=0x55c2d02eb9d0) at
postmaster.c:4437
#15 0x000055c2cfa7d0f3 in BackendStartup (port=port@entry=0x55c2d02eb9d0) at
postmaster.c:4128
#16 0x000055c2cfa7d40a in ServerLoop () at postmaster.c:1704
#17 0x000055c2cfa7e7fb in PostmasterMain (argc=3, argv=<optimized out>) at
postmaster.c:1377
#18 0x000055c2cf9d9d87 in main (argc=3, argv=0x55c2d02c4a30) at main.c:228