libxml2 2.9.3 breaks xml test output

Hello,

I just noticed that last night all built branches failed on my buildfarm
animal, jaguarundi. They all failed on the "xml" test, and the output is
essentially the same everywhere:

***************
*** 9,16 ****
LINE 1: INSERT INTO xmltest VALUES (3, '<wrong');
^
DETAIL: line 1: Couldn't find end of Start Tag wrong line 1
- <wrong
- ^
SELECT * FROM xmltest;
id | data
----+--------------------
--- 9,14 ----
***************
*** 63,70 ****
LINE 1: SELECT xmlconcat('bad', '<syntax');
^
DETAIL: line 1: Couldn't find end of Start Tag syntax line 1
- <syntax
- ^
SELECT xmlconcat('<foo/>', NULL, '<?xml version="1.1"
standalone="no"?><bar/>');
xmlconcat
--------------

The last thing I did in regard to XML on this system was upgrade libxml2
from 2.9.2 to 2.9.3. I bisected it a a bit and the guilty commit to
libxml2 is ce0b0d0d8 "Do not print error context when there is none"
from about two weeks ago.

I have zero experience with libxml2, so no idea if the previous context
level can be turned on again. IMHO, the libxml2 change is a bug in
itself; PostgreSQL's error messages are more readable with the XML text
in them.

--
Christian