Fixed inverted result code check in new TinyXML-2 code

[racko]

$ rospack profile       
terminate called after throwing an instance of 'rospack::Exception'
  what():  error parsing manifest of package rviz at /usr/share/ros_packages/rviz/package.xml

Actually XMLDocument::LoadFile returned XML_SUCCESS (== 0), but the current implementation throws an exception if LoadFile returns 0.

[dirk-thomas]

Thank you for the quick fix.