ospfd: Do not fall back to intervening router.

ospfd: Do not fall back to intervening router.

The patch in bug 330 did two things. It add a return value
whether ospf_nexthop_calculation() failed or not and also
moved the return stmt for 16.1.1 para 5 so now SPF
will fallback to the intervening router when no back links are found
by 16.1.1 para 5. This is wrong and can potentially create black holes
or routing loops according to Dave Katz and Acee Lindem at ospf@ietf.org

Even if the current code could be proved to be harmless in all cases,
it adds substantial extra processing and memory allocations.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>