BUG #17098: Assert failed on composing an error message when adding a type to an extension being dropped

The following bug has been logged on the website:

Bug reference:      17098
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 14beta2
Operating system:   Ubuntu 20.04
Description:

When trying to add to an extension a type that is already exists in the
extension while the extension is being dropped I get a failed assertion with
the following stack:
Core was generated by `postgres: law regression [local] ALTER EXTENSION
                        '.
Program terminated with signal SIGABRT, Aborted.
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50      ../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:50
#1  0x00007f59c4321859 in __GI_abort () at abort.c:79
#2  0x000055c3c74912e0 in ExceptionalCondition
(conditionName=conditionName@entry=0x55c3c76a024c "strvalue != NULL", 
    errorType=errorType@entry=0x55c3c74f000b "FailedAssertion",
fileName=0x7ffc8db80390 "\301\022I\307\303U", 
    fileName@entry=0x55c3c76a0241 "snprintf.c",
lineNumber=lineNumber@entry=442) at assert.c:69
#3  0x000055c3c74ee489 in dopr (target=target@entry=0x7ffc8db80970,
format=0x55c3c7521ee7 "\"", 
    format@entry=0x55c3c7521ec0 "%s is already a member of extension
\"%s\"", args=0x7ffc8db80a20) at snprintf.c:442
#4  0x000055c3c74eeb5c in pg_vsnprintf (str=<optimized out>,
count=<optimized out>, count@entry=1024, 
    fmt=fmt@entry=0x55c3c7521ec0 "%s is already a member of extension
\"%s\"", args=args@entry=0x7ffc8db80a20)
    at snprintf.c:195
#5  0x000055c3c74e4e33 in pvsnprintf (buf=<optimized out>,
len=len@entry=1024, 
    fmt=fmt@entry=0x55c3c7521ec0 "%s is already a member of extension
\"%s\"", args=args@entry=0x7ffc8db80a20)
    at psprintf.c:110
#6  0x000055c3c74e6221 in appendStringInfoVA (str=str@entry=0x7ffc8db80a00,

    fmt=fmt@entry=0x55c3c7521ec0 "%s is already a member of extension
\"%s\"", args=args@entry=0x7ffc8db80a20)
    at stringinfo.c:149
#7  0x000055c3c7495fbe in errmsg (fmt=fmt@entry=0x55c3c7521ec0 "%s is
already a member of extension \"%s\"")
    at elog.c:919
#8  0x000055c3c7120cda in ExecAlterExtensionContentsStmt
(stmt=stmt@entry=0x55c3c7810f60, 
    objAddr=objAddr@entry=0x7ffc8db80c14) at extension.c:3342
#9  0x000055c3c7357837 in ProcessUtilitySlow
(pstate=pstate@entry=0x55c3c7831d60, pstmt=pstmt@entry=0x55c3c7811270, 
    queryString=queryString@entry=0x55c3c7810450 "ALTER EXTENSION cube ADD
TYPE side;", 
    context=context@entry=PROCESS_UTILITY_TOPLEVEL, params=params@entry=0x0,
queryEnv=queryEnv@entry=0x0, 
    dest=0x55c3c7811340, qc=0x7ffc8db81130) at utility.c:1550
#10 0x000055c3c7356b48 in standard_ProcessUtility (pstmt=0x55c3c7811270, 
    queryString=0x55c3c7810450 "ALTER EXTENSION cube ADD TYPE side;",
readOnlyTree=<optimized out>, 
    context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0,
dest=0x55c3c7811340, qc=0x7ffc8db81130)
    at utility.c:1049
#11 0x000055c3c7356c31 in ProcessUtility (pstmt=pstmt@entry=0x55c3c7811270,
queryString=<optimized out>, 
    readOnlyTree=<optimized out>,
context=context@entry=PROCESS_UTILITY_TOPLEVEL, params=<optimized out>, 
    queryEnv=<optimized out>, dest=0x55c3c7811340, qc=0x7ffc8db81130) at
utility.c:527
#12 0x000055c3c7354157 in PortalRunUtility
(portal=portal@entry=0x55c3c78725b0, pstmt=pstmt@entry=0x55c3c7811270, 
    isTopLevel=isTopLevel@entry=true,
setHoldSnapshot=setHoldSnapshot@entry=false, dest=dest@entry=0x55c3c7811340,

    qc=qc@entry=0x7ffc8db81130) at pquery.c:1147
#13 0x000055c3c7354459 in PortalRunMulti
(portal=portal@entry=0x55c3c78725b0, isTopLevel=isTopLevel@entry=true, 
    setHoldSnapshot=setHoldSnapshot@entry=false,
dest=dest@entry=0x55c3c7811340, altdest=altdest@entry=0x55c3c7811340, 
    qc=qc@entry=0x7ffc8db81130) at pquery.c:1304
#14 0x000055c3c735488d in PortalRun (portal=portal@entry=0x55c3c78725b0,
count=count@entry=9223372036854775807, 
    isTopLevel=isTopLevel@entry=true, run_once=run_once@entry=true,
dest=dest@entry=0x55c3c7811340, 
    altdest=altdest@entry=0x55c3c7811340, qc=0x7ffc8db81130) at
pquery.c:786
#15 0x000055c3c7350ada in exec_simple_query (
    query_string=query_string@entry=0x55c3c7810450 "ALTER EXTENSION cube ADD
TYPE side;") at postgres.c:1214
#16 0x000055c3c7352aac in PostgresMain (argc=argc@entry=1,
argv=argv@entry=0x7ffc8db81320, dbname=<optimized out>, 
    username=<optimized out>) at postgres.c:4486
#17 0x000055c3c72ada9c in BackendRun (port=port@entry=0x55c3c7831220) at
postmaster.c:4507
#18 0x000055c3c72b0cb1 in BackendStartup (port=port@entry=0x55c3c7831220) at
postmaster.c:4229
#19 0x000055c3c72b0ef8 in ServerLoop () at postmaster.c:1745
#20 0x000055c3c72b2445 in PostmasterMain (argc=3, argv=<optimized out>) at
postmaster.c:1417
#21 0x000055c3c71f309e in main (argc=3, argv=0x55c3c780a4c0) at main.c:209

Reproduced with the following script:
echo "
CREATE EXTENSION cube;
DROP EXTENSION cube;
" >/tmp/ce.sql

echo "
CREATE TYPE side AS ENUM('front');
ALTER EXTENSION cube ADD TYPE side;
" >/tmp/ae.sql

for n in `seq 10`; do
  echo "iteration $n"
  ( { for f in `seq 1000`; do cat /tmp/ce.sql; done } | psql ) >psql-1.log
2>&1 &
  ( { for f in `seq 1000`; do cat /tmp/ae.sql; done } | psql ) >psql-2.log
2>&1 &
  wait
  coredumpctl --no-pager && break
done

Reproduced on REL_12_STABLE..master. The assert was added by 6d842be6c, so
on REL_11_STABLE I see just:
ERROR:  type side is already a member of extension "(null)"