Initial dialog: Fix crash with latest libxml.

	* glom/dialog_existing_or_new.cc: Catch exceptions from
	xmlpp::SaxParser::parse_chunk() because libxml nows
	returns an error code from xmlParserChunk when we call
	xmlStopParser(), which libxml++ then throws as an
	exception. Maybe libxml++ should not do that for that
	new error code.
	This is the libxml commit:
	https://git.gnome.org/browse/libxml2/commit/?id=e50ba8164eee06461c73cd8abb9b46aa0be81869