Free libxml2/libxslt resources in a safer order.

Mark Simonetti reported that libxslt sometimes crashes for him, and that
swapping xslt_process's object-freeing calls around to do them in reverse
order of creation seemed to fix it.  I've not reproduced the crash, but
valgrind clearly shows a reference to already-freed memory, which is
consistent with the idea that shutdown of the xsltTransformContext is
trying to reference the already-freed stylesheet or input document.
With this patch, valgrind is no longer unhappy.

I have an inquiry in to see if this is a libxslt bug or if we're just
abusing the library; but even if it's a library bug, we'd want to adjust
our code so it doesn't fail with unpatched libraries.

Back-patch to all supported branches, because we've been doing this in
the wrong(?) order for a long time.

Author: tglsfdc

contrib/xml2/xslt_proc.c

@@ -140,10 +140,10 @@ xslt_process(PG_FUNCTION_ARGS)
 
 	if (xslt_sec_prefs_error)
 	{
+		xsltFreeTransformContext(xslt_ctxt);
+		xsltFreeSecurityPrefs(xslt_sec_prefs);
 		xsltFreeStylesheet(stylesheet);
 		xmlFreeDoc(doctree);
-		xsltFreeSecurityPrefs(xslt_sec_prefs);
-		xsltFreeTransformContext(xslt_ctxt);
 		xsltCleanupGlobals();
 		xml_ereport(ERROR, ERRCODE_EXTERNAL_ROUTINE_EXCEPTION,
 					"could not set libxslt security preferences");
@@ -154,22 +154,22 @@ xslt_process(PG_FUNCTION_ARGS)
 
 	if (restree == NULL)
 	{
+		xsltFreeTransformContext(xslt_ctxt);
+		xsltFreeSecurityPrefs(xslt_sec_prefs);
 		xsltFreeStylesheet(stylesheet);
 		xmlFreeDoc(doctree);
-		xsltFreeSecurityPrefs(xslt_sec_prefs);
-		xsltFreeTransformContext(xslt_ctxt);
 		xsltCleanupGlobals();
 		xml_ereport(ERROR, ERRCODE_EXTERNAL_ROUTINE_EXCEPTION,
 					"failed to apply stylesheet");
 	}
 
 	resstat = xsltSaveResultToString(&resstr, &reslen, restree, stylesheet);
 
-	xsltFreeStylesheet(stylesheet);
 	xmlFreeDoc(restree);
-	xmlFreeDoc(doctree);
-	xsltFreeSecurityPrefs(xslt_sec_prefs);
 	xsltFreeTransformContext(xslt_ctxt);
+	xsltFreeSecurityPrefs(xslt_sec_prefs);
+	xsltFreeStylesheet(stylesheet);
+	xmlFreeDoc(doctree);
 
 	xsltCleanupGlobals();