bandwidth-usage.tree: Changed $PREVIOUS_TRAFFIC exit value from 1 to 0

Changed to continue the generation of the XML file, even when there's no
bandwidth usage collected data.

Signed-off-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Signed-off-by: Josiney de Souza <josineys@c3sl.ufpr.br>