Fix low-probability memory leak in XMLSERIALIZE(... INDENT).

xmltotext_with_options() did not consider the possibility that
pg_xml_init() could fail --- most likely due to OOM.  If that
happened, the already-parsed xmlDoc structure would be leaked.
Oversight in commit 483bdb2af.

Bug: #18981
Author: Dmitry Kovalenko <d.kovalenko@postgrespro.ru>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18981-9bc3c80f107ae925@postgresql.org
Backpatch-through: 16

src/backend/utils/adt/xml.c

@@ -663,7 +663,7 @@ xmltotext_with_options(xmltype *data, XmlOptionType xmloption_arg, bool indent)
 	volatile xmlBufferPtr buf = NULL;
 	volatile xmlSaveCtxtPtr ctxt = NULL;
 	ErrorSaveContext escontext = {T_ErrorSaveContext};
-	PgXmlErrorContext *xmlerrcxt;
+	PgXmlErrorContext *volatile xmlerrcxt = NULL;
 #endif
 
 	if (xmloption_arg != XMLOPTION_DOCUMENT && !indent)
@@ -704,13 +704,18 @@ xmltotext_with_options(xmltype *data, XmlOptionType xmloption_arg, bool indent)
 		return (text *) data;
 	}
 
-	/* Otherwise, we gotta spin up some error handling. */
+	/*
-	xmlerrcxt = pg_xml_init(PG_XML_STRICTNESS_ALL);
+	 * Otherwise, we gotta spin up some error handling.  Unlike most other
-
+	 * routines in this module, we already have a libxml "doc" structure to
+	 * free, so we need to call pg_xml_init() inside the PG_TRY and be
+	 * prepared for it to fail (typically due to palloc OOM).
+	 */
 	PG_TRY();
 	{
 		size_t		decl_len = 0;
 
+		xmlerrcxt = pg_xml_init(PG_XML_STRICTNESS_ALL);
+
 		/* The serialized data will go into this buffer. */
 		buf = xmlBufferCreate();
 
@@ -838,9 +843,9 @@ xmltotext_with_options(xmltype *data, XmlOptionType xmloption_arg, bool indent)
 			xmlSaveClose(ctxt);
 		if (buf)
 			xmlBufferFree(buf);
-		if (doc)
 		xmlFreeDoc(doc);
 
+		if (xmlerrcxt)
 			pg_xml_done(xmlerrcxt, true);
 
 		PG_RE_THROW();