pimd: clear rp_info source_nexthop when RP becomes not reachable

pimd: clear rp_info source_nexthop when RP becomes not reachable

When route to RP gets modified, FRR receives a notification from
zebra, and call the function pim_update_rp_nh() to compute the
new nexthop and will update the source_nexthop information of
rp_info. This is not working for the case when RP becomes not
reachable.

Fix: When FRR receives a notification from zebra saying RP becomes
not reachable, then delete the source_nexthop informatio of rp_info.

Signed-off-by: Sarita Patra <saritap@vmware.com>