gnrc_ipv6: fix hop-limit == 0 case
gnrc_ipv6: fix hop-limit == 0 case

RIOT-OS/RIOT,kaspar030/RIOT,basilfx/RIOT,authmillenon/RIOT,basilfx/RIOT,toonst/RIOT,OTAkeys/RIOT,yogo1212/RIOT,BytesGalore/RIOT,cladmi/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,mfrey/RIOT,RIOT-OS/RIOT,smlng/RIOT,BytesGalore/RIOT,rfuentess/RIOT,miri64/RIOT,smlng/RIOT,miri64/RIOT,OlegHahm/RIOT,aeneby/RIOT,gebart/RIOT,aeneby/RIOT,authmillenon/RIOT,toonst/RIOT,jasonatran/RIOT,authmillenon/RIOT,kaspar030/RIOT,mtausig/RIOT,mfrey/RIOT,kaspar030/RIOT,miri64/RIOT,OTAkeys/RIOT,aeneby/RIOT,lazytech-org/RIOT,A-Paul/RIOT,lazytech-org/RIOT,OlegHahm/RIOT,rfuentess/RIOT,rfuentess/RIOT,OTAkeys/RIOT,A-Paul/RIOT,BytesGalore/RIOT,kYc0o/RIOT,toonst/RIOT,aeneby/RIOT,mfrey/RIOT,jasonatran/RIOT,mtausig/RIOT,aeneby/RIOT,x3ro/RIOT,cladmi/RIOT,x3ro/RIOT,gebart/RIOT,ant9000/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,yogo1212/RIOT,lazytech-org/RIOT,kYc0o/RIOT,josephnoir/RIOT,basilfx/RIOT,basilfx/RIOT,kYc0o/RIOT,ant9000/RIOT,BytesGalore/RIOT,yogo1212/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,BytesGalore/RIOT,A-Paul/RIOT,yogo1212/RIOT,authmillenon/RIOT,mtausig/RIOT,cladmi/RIOT,gebart/RIOT,RIOT-OS/RIOT,x3ro/RIOT,rfuentess/RIOT,josephnoir/RIOT,mfrey/RIOT,mtausig/RIOT,lazytech-org/RIOT,kaspar030/RIOT,smlng/RIOT,x3ro/RIOT,gebart/RIOT,OTAkeys/RIOT,A-Paul/RIOT,mtausig/RIOT,smlng/RIOT,josephnoir/RIOT,x3ro/RIOT,cladmi/RIOT,yogo1212/RIOT,jasonatran/RIOT,OlegHahm/RIOT,rfuentess/RIOT,toonst/RIOT,authmillenon/RIOT,cladmi/RIOT,miri64/RIOT,ant9000/RIOT,kYc0o/RIOT,kYc0o/RIOT,gebart/RIOT,OTAkeys/RIOT,mfrey/RIOT,basilfx/RIOT,kaspar030/RIOT,miri64/RIOT,yogo1212/RIOT,smlng/RIOT,josephnoir/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,toonst/RIOT,jasonatran/RIOT,A-Paul/RIOT