gnrc_ipv6_nib: get node from proper interface
gnrc_ipv6_nib: get node from proper interface

If we switch the interface in gnrc_ipv6_nib_get_next_hop_l2addr()
we must also re-get the nib entry from the 'proper' interface.
Otherwise we will always find the host unreachable on the 'wrong'
interface.

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