pimd: crash fix when RP is removed

pimd: crash fix when RP is removed

pimd crash at pim_msg_build_jp_groups (
grp=grp@entry=0x7ffca55b5d1e, sgs=sgs@entry=0x17821a0, size=20)
 at pimd/pim_msg.c:198

Fix for https://github.com/FRRouting/frr/issues/6849

Root Cause:
===========
pimd has crashed because pim_upstream_rpf_clear function sets the
up->rpf.source_nexthop.interface pointer to NULL and has not removed
the upstream source node from the neighbor. When the upstream gets
deleted the source is not removed from neighbor
neigh->upstream_jp_agg->groups->sources list. This source node has
pointer to upstream freed memory. Hence when on_neighbor_jp_timer expires,
it tries to access the upstream pointer and crashed.

Fix:
====
Before setting the interface pointer to NULL, remove the node from
neigh->upstream_jp_agg->groups->sources list. Also the upstream state
has to be changed to Not joined.

Removed extra line changes.

Signed-off-by: Mobashshera Rasool <mrasool@vmware.com>