xml parsing: add XML_PARSE_HUGE flag to xmlReadMemory()

It looks like libxml2 has some internal limitations by default that
causes parse failures in some situations.  Avoid them with
XML_PARSE_HUGE.

Without this, you get errors like

    test.xml:349250: parser error : internal error: Huge input lookup
    όμουν τουλάχιστον αλλά +2kg και ενδεχομένως +4
                                                                                   ^
when something in the xml file grows too large.

I don't know libxml2 internals, so I have no idea what exactly goes
wrong, but the docs say:

    XML_PARSE_HUGE = 524288 : relax any hardcoded limit from the parser

and that makes us successfully parse the Greek file from Kostas.

Reported-by: Kostas Katsioulis <kostaskatsioulis@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

core/parse-xml.c

@@ -1748,9 +1748,9 @@ int parse_xml_buffer(const char *url, const char *buffer, int size,
 	state.sites = sites;
 	state.devices = devices;
 	state.filter_presets = filter_presets;
-	doc = xmlReadMemory(res, strlen(res), url, NULL, 0);
+	doc = xmlReadMemory(res, strlen(res), url, NULL, XML_PARSE_HUGE);
 	if (!doc)
-		doc = xmlReadMemory(res, strlen(res), url, "latin1", 0);
+		doc = xmlReadMemory(res, strlen(res), url, "latin1", XML_PARSE_HUGE);
 
 	if (res != buffer)
 		free((char *)res);

core/save-xml.c

@@ -871,7 +871,7 @@ static int export_dives_xslt_doit(const char *filename, struct xml_params *param
 	 * transform it to selected export format, finally dumping
 	 * the XML into a character buffer.
 	 */
-	doc = xmlReadMemory(buf.buffer, buf.len, "divelog", NULL, 0);
+	doc = xmlReadMemory(buf.buffer, buf.len, "divelog", NULL, XML_PARSE_HUGE);
 	free_buffer(&buf);
 	if (!doc)
 		return report_error("Failed to read XML memory");