SimXMLDocument GetData() will crash if it encounters an empty XML elemen...

...t

This bug was submitted 3 years ago but has never been fixed.
http://www.garagegames.com/community/forums/viewthread/120911

In some cases,  FirstChild() can be null, giving a crash when we call ToText().


Here's a simple example to reproduce the crash

```c++
//Parse the xml => Go to "tag" element and get data
	%xml = "<xml><tag/></xml>";
	%parser = new SimXMLDocument();
	%parser.parse(%xml);
	%parser.pushChildElement(0);
	%parser.pushChildElement(0);
	echo("Current element is:" SPC %parser.elementValue());	//To check that the current tag is "tag"
	echo("Data:" SPC  %parser.getData());
```