fib: Fix crash when removing a covering prefix

Type: fix

When a covering entry is removed from the table, the covered entries first see it 'updated' and then 'removed'.
the crash occurs because the covered prefixes share (simple pointer copy) the covereds hash table of path extensions. During the cervers deletion this hash table has been removed and the update of the covered crashes when recaluationg forwarding becuase it uses the free'd hash.
Fix is to refetch the shared hash table (which is NULL) when the covered is updated.

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: Icefca9d7b21da975111d0e974d75f663fc0cc00c