OSPF: Fix net-summary origination combined with stubnet option

Stubnet nodes in OSPF FIB were removed during rt_sync(), but the pointer
remained in top_hash_entry.nf, so net-summary LSA origination was
confused, reported 'LSA ID collision' and net-summary LSAs were not
originated properly.

Thanks to Naveen Chowdary Yerramneni for bugreport and analysis.