BUG #16953: OOB access while converting "interval" to char

The following bug has been logged on the website:

Bug reference:      16953
Logged by:          Theodor Arsenij Larionov-Trichkin
Email address:      t.larionov@postgrespro.ru
PostgreSQL version: 13.2
Operating system:   Ubuntu 20.04.2 LTS
Description:        

Hello!
How to reproduce:
1. mkdir -p ./installation/databases
2. git clone --single-branch --depth=1 --branch=REL_13_2
https://github.com/postgres/postgres postgres_src
3. cd postgres_src
4. ./configure --prefix=`pwd`/../installation/pgbuild
5. make -j20 && make install && cd ..
6. ./installation/pgbuild/bin/initdb -U username -D
./installation/databases/db_clean
7. ./installation/pgbuild/bin/postgres -D
./installation/databases/db_clean/
8. ./installation/pgbuild/bin/psql -h 127.0.0.1 -p 5432 -U username
postgres
9. Performing this query will result in OOB access of rm_months_lower array
and as a result crash: SELECT * from TO_CHAR(interval '-1Mon', 'rm');

Output:
2021-04-07 12:07:27.060 MSK [33887] LOG:  starting PostgreSQL 13.2 on
x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0,
64-bit
2021-04-07 12:07:27.060 MSK [33887] LOG:  listening on IPv4 address
"127.0.0.1", port 5432
2021-04-07 12:07:27.065 MSK [33887] LOG:  listening on Unix socket
"/tmp/.s.PGSQL.5432"
2021-04-07 12:07:27.069 MSK [33888] LOG:  database system was shut down at
2021-04-07 12:07:22 MSK
2021-04-07 12:07:27.071 MSK [33887] LOG:  database system is ready to accept
connections
2021-04-07 12:08:01.013 MSK [33887] LOG:  server process (PID 34113) was
terminated by signal 11: Segmentation fault
2021-04-07 12:08:01.013 MSK [33887] DETAIL:  Failed process was running:
SELECT * from TO_CHAR(interval '-1Mon', 'rm');
2021-04-07 12:08:01.013 MSK [33887] LOG:  terminating any other active
server processes
2021-04-07 12:08:01.013 MSK [33892] WARNING:  terminating connection because
of crash of another server process
2021-04-07 12:08:01.013 MSK [33892] DETAIL:  The postmaster has commanded
this server process to roll back the current transaction and exit, because
another server process exited abnormally and possibly corrupted shared
memory.
2021-04-07 12:08:01.013 MSK [33892] HINT:  In a moment you should be able to
reconnect to the database and repeat your command.
2021-04-07 12:08:01.013 MSK [35036] FATAL:  the database system is in
recovery mode
2021-04-07 12:08:01.014 MSK [33887] LOG:  all server processes terminated;
reinitializing
2021-04-07 12:08:01.027 MSK [35038] LOG:  database system was interrupted;
last known up at 2021-04-07 12:07:27 MSK
2021-04-07 12:08:01.248 MSK [35038] LOG:  database system was not properly
shut down; automatic recovery in progress
2021-04-07 12:08:01.249 MSK [35038] LOG:  redo starts at 0/1559798
2021-04-07 12:08:01.249 MSK [35038] LOG:  invalid record length at
0/15597D0: wanted 24, got 0
2021-04-07 12:08:01.249 MSK [35038] LOG:  redo done at 0/1559798
2021-04-07 12:08:01.256 MSK [33887] LOG:  database system is ready to accept
connections

Postgres version:
PostgreSQL 13.2 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
9.3.0-17ubuntu1~20.04) 9.3.0, 64-bit