sliptty/start_network.sh: fix if no argument is given
sliptty/start_network.sh: fix if no argument is given

fix for 'shift: can't shift that many'

OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,jasonatran/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,yogo1212/RIOT,OTAkeys/RIOT,basilfx/RIOT,kaspar030/RIOT,miri64/RIOT,basilfx/RIOT,yogo1212/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,yogo1212/RIOT,authmillenon/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,yogo1212/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,basilfx/RIOT,authmillenon/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,basilfx/RIOT,OlegHahm/RIOT,jasonatran/RIOT,basilfx/RIOT,miri64/RIOT,yogo1212/RIOT,kYc0o/RIOT,OlegHahm/RIOT,yogo1212/RIOT