ospfd: Prevent use after free( and crash of ospf ) when no router ospf

ospfd: Prevent use after free( and crash of ospf ) when no router ospf

Consider this config:

router ospf
  redistribute kernel

Then you issue:

no router ospf

ospf will crash with a use after free.

The problem is that the event's associated with the
ospf pointer were shut off then the ospf_external_delete
was called which rescheduled the event.  Let's just move
event deletion to the end of the no router ospf.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>