BUG #17951: hashtext('input') returning non-integer value for certain inputs

The following bug has been logged on the website:

Bug reference:      17951
Logged by:          Ilkka Kaakkola
Email address:      ilkka.kaakkola+github@gmail.com
PostgreSQL version: 15.3
Operating system:   Linux (Docker)
Description:

Startup command:

user@host % docker run --rm -d --name psql -e POSTGRES_PASSWORD=password -e
POSTGRES_DB=app postgres:15.3 postgres
028d012d418351520815376f2111d47c105bfa0a0dfcc95125dbcacc3318c7d6

user@host % docker exec -it psql bash

(inside container)
root@028d012d4183:/# uname -a
Linux 028d012d4183 5.15.96-0-virt #1-Alpine SMP Sun, 26 Feb 2023 15:14:12
+0000 x86_64 GNU/Linux

root@028d012d4183:/# psql -U postgres app
psql (15.3 (Debian 15.3-1.pgdg110+1))
Type "help" for help.

app=#


PostgreSQL version:

app=# select VERSION();
                                                           version

-----------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 15.3 (Debian 15.3-1.pgdg110+1) on x86_64-pc-linux-gnu, compiled
by gcc (Debian 10.2.1-6) 10.2.1 20210110, 64-bit
(1 row)

----

Steps to reproduce:

app=# CREATE TABLE bug ( id varchar(128) );
CREATE TABLE


app=# \d bug
                        Table "public.bug"
 Column |          Type          | Collation | Nullable | Default
--------+------------------------+-----------+----------+---------
 id     | character varying(128) |           |          |


app=# insert into bug (id) values ('18122447326473c034-214042692253025');
INSERT 0 1


app=# select ABS(HASHTEXT(id)) FROM bug;
ERROR:  22003: integer out of range
LOCATION:  int4abs, int.c:1196

The resulting value of hashtext() for the given input overflows an integer
by 1 as can be seen by casting it manually

app=# select ABS(CAST(HASHTEXT(ID) as BIGINT)) FROM bug;
    abs
------------
 2147483648
(1 row)


This can also be directly observed by selecting
HASHTEXT('18122447326473c034-214042692253025'):

app=# SELECT HASHTEXT('18122447326473c034-214042692253025');
  hashtext
-------------
 -2147483648
(1 row)

Expected the output from hashtext('input') to always be a valid integer.