bgpd: Avoid memset when tip hash is empty

bgpd: Avoid memset when tip hash is empty

The tip hash is only used when we are dealing with
evpn.  In bgp_nexthop_self we are doing a memset
irrelevant of whether we will ever find data.  Yes
hash_lookup will return pretty quickly.

Modify the code to avoid doing a memset in the case
where the tip hash is empty as that we know we'll
never find anything.  With full BGP feeds this
small memset does take some time.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>