sc_gnrc_ipv6_nib: check interface existence
sc_gnrc_ipv6_nib: check interface existence

Currently an interface's existence is not checked when it is supplied
by the user with the `nib` command. This can lead to assertion errors
as soon as the generated entry tries to resolve an address or route
generated with that command and the network interface not being found.

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