ipv6: check return value of ipv6_get_lladdr

We should check the return value of ipv6_get_lladdr in inet6_set_iftoken.

A possible situation, which could leave ll_addr unassigned is, when
the user removed her link-local address but a global scoped address was
already set. In this case the interface would still be IF_READY and not
dead. In that case the RS source address is some value from the stack.

v2: Daniel Borkmann noted a small indent inconstancy; no semantic
changes.

Cc: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Reviewed-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>