zebra: stop sending invalid nexthops to clients

zebra: stop sending invalid nexthops to clients

Found that zebra_rnh_apply_nht_rmap would set the
NEXTHOP_FLAG_ACTIVE if not blocked by the route-map, even
if the flag was not active prior to the check.  This fix
changes the flag used to denote the nexthop is filtered so
that proper active state can be retained. Additionally,
found two cases where we would send invalid nexthops via
send_client, which would also cause this crash.  All three
fixed in this commit.

Signed-off-by: Don Slice <dslice@cumulusnetworks.com>