pimd: Fix nexthop determination when sending towards RP

pimd: Fix nexthop determination when sending towards RP

When sending a PIM join upwards on the RP-based tree, it may get dropped on
the last hop before the RP if the RP is reachable via a connected route
(i.e. there's no associated nexthop). pimd needs to put the nexthop IP
address into the PIM join payload and fails to do that if that route has a
nexthop of 0.0.0.0. So whenever we look up a route to determine the nexthop
or we receive a nexthop tracking update from Zebra, use the destination
address as the nexthop address for connected routes.

Fixes #2326.

Signed-off-by: Martin Buck <mb-tmp-tvguho.pbz@gromit.dyndns.org>