[Cooja] plugins/LogListener: Prevent from throwing ArrayIndexOutOfBoundsException

When you click on the *Mote* column in an empty `LogListener` plugin
an uncaught `ArrayIndexOutOfBoundsException` is thrown.

As I always hear all alarm bells ringing when seeing a stacktrace in the Cooja log I would suggest preventing this exception by checking that `rowIndex` is `!= -1 before using it.