tdf#136551 DOTX import: restore support for large XML attribute values

Regression from commit 82d08580e3 (switch
saxparser from expat to libxml2, 2014-11-14), expat used to allow huge
XML attribute values, while libxml2 defaults to rejecting values larger
than 10MB.

This looks like a sane limit, but the bugdoc has some fallback VML
markup where the actual graphic content of the shape is base64-encoded
in an XML attribute value.

libxml2 has an XML_PARSE_HUGE flag to lift this limit, so use that. If
this was not a problem with expat, then it should be no problem with
libxml2, either.

[ No testcase, adding a 10MB test document to the repo is not preferred. ]

Change-Id: Ifcd0ce52d3cb95bef36c58aa073bb59bc07490d6
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/103567
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

sax/source/fastparser/fastparser.cxx

@@ -1061,8 +1061,10 @@ void FastSaxParserImpl::parse()
                 throw SAXException("Couldn't create parser", Reference< XInterface >(), Any() );
 
             // Tell libxml2 parser to decode entities in attribute values.
+            // Also allow XML attribute values which are larger than 10MB, because this used to work
+            // with expat.
             // coverity[unsafe_xml_parse_config] - entity support is required
-            xmlCtxtUseOptions(rEntity.mpParser, XML_PARSE_NOENT);
+            xmlCtxtUseOptions(rEntity.mpParser, XML_PARSE_NOENT | XML_PARSE_HUGE);
         }
         else
         {