[ospfd] Fix SEGV during startup introduced in recent commit

[ospfd] Fix SEGV during startup introduced in recent commit

* ospfd/ospfd.c: (opsf_if_update) Check if ospf_lookup() returns NULL.
  Fixes bug introduced with recent ospf_network_run changes in
  a49eb30a, where SEGV will happen if not the "router ospf" command has
  been executed before the first interfaces are reported by zebra.

Signed-off-by: Paul Jakma <paul@quagga.net>