bgpd: Prevent possible crash when parsing v6 attributes

bgpd: Prevent possible crash when parsing v6 attributes

The peer->nexthop.ifp pointer must be set when parsing the
attributes in bgp_mp_reach_parse, notice this
and fail gracefully.

Rework bgp_nexthop_set to remove the HAVE_CUMULUS and to
fail the nexthop_set when we have a zebra connection and
no ifp pointer, as that not havinga zebra connection and
no ifp pointer is legal.

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