gnrc_ipv6: fix source check for loopback address
gnrc_ipv6: fix source check for loopback address

When the destination address is the loopback address (`::1`) in GNRC
the selected network interface typically is `NULL`, as with GNRC no
loopback interface de facto exists. So the assertion when checking if
the source address is valid if `netif != NULL` fails on that check.
This change fixes that issue by checking if the destination address is
the loopback address, before checking the validity of the source
address.

toonst/RIOT,smlng/RIOT,jasonatran/RIOT,OlegHahm/RIOT,ant9000/RIOT,mtausig/RIOT,ant9000/RIOT,smlng/RIOT,OlegHahm/RIOT,miri64/RIOT,josephnoir/RIOT,basilfx/RIOT,authmillenon/RIOT,basilfx/RIOT,basilfx/RIOT,ant9000/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,aeneby/RIOT,mtausig/RIOT,basilfx/RIOT,kYc0o/RIOT,x3ro/RIOT,smlng/RIOT,yogo1212/RIOT,mtausig/RIOT,smlng/RIOT,kYc0o/RIOT,basilfx/RIOT,kYc0o/RIOT,x3ro/RIOT,kaspar030/RIOT,toonst/RIOT,yogo1212/RIOT,miri64/RIOT,toonst/RIOT,x3ro/RIOT,kaspar030/RIOT,OlegHahm/RIOT,miri64/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,yogo1212/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,josephnoir/RIOT,josephnoir/RIOT,josephnoir/RIOT,OlegHahm/RIOT,jasonatran/RIOT,josephnoir/RIOT,aeneby/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,aeneby/RIOT,kaspar030/RIOT,miri64/RIOT,yogo1212/RIOT,authmillenon/RIOT,OTAkeys/RIOT,yogo1212/RIOT,aeneby/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,mtausig/RIOT,toonst/RIOT,smlng/RIOT,x3ro/RIOT,ant9000/RIOT,mtausig/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,jasonatran/RIOT,aeneby/RIOT,authmillenon/RIOT,toonst/RIOT,x3ro/RIOT,yogo1212/RIOT