tmf : Fix potential null pointer dereference in XmlUtils

tmf : Fix potential null pointer dereference in XmlUtils

There is a possible null pointer exception that would have happened if
I/O errors occur.

Change-Id: I89bde3aaf4d82c71fb46fe6b4e9720603ad481bf
Signed-off-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/66347
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>