gnrc_ipv6_nib: fix RA reception for non-6LN interfaces

When having a non-6LN interface and a 6LN interface (e.g. on a border
router) the assertion can hit when a Router Advertisement is received.
This makes the check an `if` statement rather than an assertion, to
account for that case.

Co-authored-by: Gunar Schorcht <gunar@schorcht.net>