ospfd: fix memory leak on SPF calculation

ospfd: fix memory leak on SPF calculation

Fix the following problems:
- Always free vertex next hops on `vertex_parent_free`
- Signalize failure on `ospf_spf_add_parent` when parent already exists
  so the caller has the chance to `free()` any allocated resources.
- Don't reuse vertex next hops without the reference count logic in
  `ospf_nexthop_calculation`. Instead allocate a new copy so it can be
  `free()`d later without complications

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>