BUG #15729: Error 'invalid memory alloc request size' during updating a big xml field

The following bug has been logged on the website:

Bug reference: 15729
Logged by: Kent
Email address: varuzam(at)gmail(dot)com
PostgreSQL version: 11.2
Operating system: Centos 7 64bit
Description:

Hello
I have a table with big records
```
test=# \d xrecords
Table "public.xrecords"
Column | Type | Collation | Nullable | Default
--------+---------------+-----------+----------+---------
recid | character(19) | | not null |
detail | xml | | |
data | oid | | |
Indexes:
"recidkey" PRIMARY KEY, btree (recid)
"delete_ix" btree (text((xpath('/detail/delete_date/text()'::text,
detail))[1]))
Triggers:
delete_all_trigger BEFORE TRUNCATE ON xrecords FOR EACH STATEMENT
EXECUTE PROCEDURE clean_all_data()
delete_trigger BEFORE DELETE ON xrecords FOR EACH ROW EXECUTE PROCEDURE
clean_data()
insert_or_update_trigger BEFORE INSERT ON xrecords FOR EACH ROW EXECUTE
PROCEDURE insert_or_update()
update_trigger BEFORE UPDATE ON xrecords FOR EACH ROW EXECUTE PROCEDURE
clean_data()

test=# select length(detail::text) from xrecords WHERE
recid='GU20190309070000020' ;
length
-----------
493514189
```

The error occurs during this query
```
test=# UPDATE xrecords SET detail = XMLPARSE (DOCUMENT
replace(text(detail),'<update_date>','<delete_date>2019-04-01
10:04:05</delete_date><update_date>')) WHERE recid='GU20190309070000020' AND
text((xpath('/detail/delete_date/text()', detail))[1]) IS NULL;

ERROR: invalid memory alloc request size 1974056760
```

is it a bug?