Fix negative offset conversion bug

Author: GTO2013

src/main/java/com/romraider/xml/ConversionLayer/XDFConversionLayer.java

@@ -516,7 +516,7 @@ private Node parseXDFHeader(Document doc, Node romNode, Node header) {
         idAddress.setTextContent("-1");
 
         Element offsetNode = doc.createElement("offset");
-        offsetNode.setTextContent("0x" + Integer.toHexString(offset));
+        offsetNode.setTextContent("0x" + Integer.toString(offset, 16));
 
         romIDNode.appendChild(offsetNode);
         romIDNode.appendChild(idAddress);