pimd: RPF change to unreachable was leaving a stale entry in the jp-agg list

pimd: RPF change to unreachable was leaving a stale entry in the jp-agg list

This was causing pimd to crash later; call-stack -
(gdb) bt
    context=<optimized out>) at lib/sigevent.c:254
    group=group@entry=0x7ffffa9797e0) at pimd/pim_rp.c:207
    grp=grp@entry=0x7ffffa9799fe, sgs=sgs@entry=0x560ac069edb0, size=52)
    at pimd/pim_msg.c:200
    groups=<optimized out>) at pimd/pim_join.c:562
    at pimd/pim_neighbor.c:288
    at lib/thread.c:1599
    at lib/libfrr.c:1024
    envp=<optimized out>) at pimd/pim_main.c:162
(gdb) fr 4
    group=group@entry=0x7ffffa9797e0) at pimd/pim_rp.c:207
207     pimd/pim_rp.c: No such file or directory.
(gdb) fr 6
    grp=grp@entry=0x7ffffa9799fe, sgs=sgs@entry=0x560ac069edb0, size=52)
    at pimd/pim_msg.c:200
200     pimd/pim_msg.c: No such file or directory.
(gdb) p source->up->sg_str
$1 = '\000' <repeats 31 times>, <incomplete sequence \361>
(gdb)

This problem can manifest in the following event sequence -
1. upstream RPF neighbor is resolved
2. upstream RPF neighbor becomes unresolved (but upstream entry
   stays on the jp-agg list)
3. upstream entry is removed
on the next old-neighbor jp-agg-list processing the stale entry is
accessed resulting in the crash.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>