Bugfix: avoid sending bogus data over the radio when no prefix is set; bug reported by mathieupouillot
Bugfix: avoid sending bogus data over the radio when no prefix is set; bug reported by mathieupouillot

MohamedSeliem/contiki,arurke/contiki,bluerover/6lbr,arurke/contiki,arurke/contiki,bluerover/6lbr,arurke/contiki,arurke/contiki,bluerover/6lbr,bluerover/6lbr,arurke/contiki,bluerover/6lbr,MohamedSeliem/contiki,bluerover/6lbr,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr