gnrc_netif_lorawan: add missing NULL check
gnrc_netif_lorawan: add missing NULL check

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