[libxml++] Return value of SaxParser::on_get_entity()

[Jonathan Wakely <cow compsoc man ac uk>]

Hi all,

I raised something on the patch manager at sourceforge, and as I'll be
offline until sunday and won't be able to dig deeper I wanted to raise
it here and see if anyone knows the right answer.
http://sourceforge.net/tracker/index.php?func=detail&aid=708826&group_id=12999&atid=312999

Basically I'm not sure the on_get_entity() callback I added to
SaxParser is doing the right thing.

As I'd been overriding this function in my code I'd not thought about
the default case carefully enough, but I think just doing a lookup
with xmlGetPredefinedEntity() is wrong.

I think it should return NULL, so that libxml2 will fall back to doing
the lookup (or maybe something else if it changes in future).

The relevant libxml code is xmlParseEntityRef() in parser.c line 5800

When I'm back I'll look further,

jon