Suppress "chunk is not well balanced" errors from libxml2.

libxml2 2.13 has an entirely different rule than earlier versions
about when to emit "chunk is not well balanced" errors.  This
causes regression test output discrepancies for three test cases
that formerly provoked that error (along with others) and now don't.

Closer inspection shows that at least in 2.13, this error is pretty
useless because it can only be emitted after some other more-relevant
error.  So let's get rid of the cross-version discrepancy by just
suppressing it.  In case some older libxml2 version is capable of
emitting this error by itself, suppress only when some other error
has already been captured.

Like 066e8ac6e and 6082b3d5d, this will need to be back-patched,
but let's check the results in HEAD first.  (The patch for xml_2.out,
in particular, is blind since I can't test it here.)

Erik Wienhold and Tom Lane, per report from Frank Streitzig.

Discussion: https://postgr.es/m/trinity-b0161630-d230-4598-9ebc-7a23acdb37cb-1720186432160@3c-app-gmx-bap25
Discussion: https://postgr.es/m/trinity-361ba18b-541a-4fe7-bc63-655ae3a7d599-1720259822452@3c-app-gmx-bs01

src/backend/utils/adt/xml.c

@@ -2114,6 +2114,19 @@ xml_errorHandler(void *data, PgXmlErrorPtr error)
 	switch (domain)
 	{
 		case XML_FROM_PARSER:
+
+			/*
+			 * XML_ERR_NOT_WELL_BALANCED is typically reported after some
+			 * other, more on-point error.  Furthermore, libxml2 2.13 reports
+			 * it under a completely different set of rules than prior
+			 * versions.  To avoid cross-version behavioral differences,
+			 * suppress it so long as we already logged some error.
+			 */
+			if (error->code == XML_ERR_NOT_WELL_BALANCED &&
+				xmlerrcxt->err_occurred)
+				return;
+			/* fall through */
+
 		case XML_FROM_NONE:
 		case XML_FROM_MEMORY:
 		case XML_FROM_IO: