zebra: Fix zebra crashed in building FPM netlink message when bgp sends aggregation...

zebra: Fix zebra crashed in building FPM netlink message when bgp sends aggregation routes to zebra.

Issue:
    When BGP sends aggregation routes to zebra, the next hop is black hole.
    Then Zebra will try to build the netlink FPM message, but there is no
    next hop as it is a black hole route. Then the netlink_route_info_fill
    function returns 0. In the result, zebra will crashed in
    "assert(data_len)" of zfpm_build_route_updates.
    This issue also happen when I create a static black hole route via
    staticd.
Fix:
    As the netlink message of the blackhole route is legal, it should return
    success.

Signed-off-by: Richard Wu <wutong23@baidu.com>