fib: fix crash on exporter tracker remove

fib: fix crash on exporter tracker remove

Exported entries are tracked only when the prefix found in the export
FIB is really attached, exporter tracker is not set if the export
entry is not valid for export, ex. for special FIB entries - default
route, zeronet, mcast and broadcast prefixes.
When imported entries need to be purged, such unset exporter tracker is
being removed by non-initialized index with absent delegate entries,
causing corresponding assert and crash.

Type: fix
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>
Change-Id: Ib24a2e7853a03a960577872480213e1e8097da5a