BUG #15684: Server crash on DROP partitioned table

The following bug has been logged on the website:

Bug reference:      15684
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 11.2
Operating system:   Ubuntu 18.04
Description:

The following query:
create table at_partitioned (a int, b text) partition by range (a);
create table at_part_1 partition of at_partitioned for values from (0) to
(1000);
create table at_part_2 partition of at_partitioned for values from (1000) to
(2000);
create index on at_partitioned (b);
alter table at_partitioned alter column b type numeric using b::numeric;
alter table at_partitioned alter column b type numeric using b::numeric;
drop table at_partitioned cascade;

crashes server (on REL_11_2 and REL_11_STABLE) with the error messages:
psql:query.sql:7: WARNING:  AbortTransaction while in COMMIT state
psql:query.sql:7: ERROR:  SMgrRelation hashtable corrupted
PANIC:  cannot abort transaction 575, it was already committed
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
psql:query.sql:7: connection to server was lost

and the following stack trace:
Core was generated by `postgres: law regression [local] DROP TABLE
                        '.
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  0x00007f715f347801 in __GI_abort () at abort.c:79
#2  0x00005624ec109351 in errfinish (dummy=dummy@entry=0) at elog.c:555
#3  0x00005624ec10b1c6 in elog_finish (elevel=elevel@entry=22, 
    fmt=fmt@entry=0x5624ec195190 "cannot abort transaction %u, it was
already committed") at elog.c:1376
#4  0x00005624ebd63826 in RecordTransactionAbort
(isSubXact=isSubXact@entry=false) at xact.c:1580
#5  0x00005624ebd63942 in AbortTransaction () at xact.c:2602
#6  0x00005624ebd64385 in AbortCurrentTransaction () at xact.c:3144
#7  0x00005624ebfeba10 in PostgresMain (argc=<optimized out>,
argv=argv@entry=0x5624ece2d748, dbname=<optimized out>, 
    username=0x5624ecdfeab8 "law") at postgres.c:3968
#8  0x00005624ebcc133d in BackendRun (port=0x5624ece25e40) at
postmaster.c:4361
#9  BackendStartup (port=0x5624ece25e40) at postmaster.c:4033
#10 ServerLoop () at postmaster.c:1706
#11 0x00005624ebf6a668 in PostmasterMain (argc=3, argv=0x5624ecdfc9f0) at
postmaster.c:1379
#12 0x00005624ebcc2f69 in main (argc=3, argv=0x5624ecdfc9f0) at main.c:228