ospfd: avoid exhausting memory with OSPF vertices (BZ#476)

ospfd: avoid exhausting memory with OSPF vertices (BZ#476)

This was found in scale testing at OSR;  ospfd is adding the same link
over and over again to the SPF tree.  This fix prevents the resulting
memory corruption from happening and adds a debug message to track
occurence of this issue and/or confirm a proper fix.

(This version was improved by Scott Feldman over the earlier RFC.)

* ospfd/ospf_spf.c: (ospf_spf_add_parent) loop over existing vertices
  and refuse to add duplicates.

Tested-by: Martin Winter <mwinter@opensourcerouting.org>
Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>