Minor Fix for DJ-ImageSlider

Some of you struggled with the Notice visible in the DJ-ImageSlider module: 

"Undefined property: stdClass::$rel" [...]

This updated version brings a fix for it.

For the record: Please note that this was only a notice message and that these kinds of messages are not intended for productive websites but for software developers only (they don't harm the module logic at all). You should change error reporting on your productive websites to simple or none in the global Joomla configuration.