uhcp: fix handling of pushed prefix
uhcp: fix handling of pushed prefix

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