pimd: Fix use after free issue for ifp's moving vrfs

pimd: Fix use after free issue for ifp's moving vrfs

We have this valgrind trace:

==1125== Invalid read of size 4
==1125==    at 0x170A7D: pim_if_delete (pim_iface.c:203)
==1125==    by 0x170C01: pim_if_terminate (pim_iface.c:80)
==1125==    by 0x174F34: pim_instance_terminate (pim_instance.c:68)
==1125==    by 0x17535B: pim_vrf_terminate (pim_instance.c:260)
==1125==    by 0x1941CF: pim_terminate (pimd.c:161)
==1125==    by 0x1B476D: pim_sigint (pim_signals.c:44)
==1125==    by 0x4910C22: frr_sigevent_process (sigevent.c:133)
==1125==    by 0x49220A4: thread_fetch (thread.c:1777)
==1125==    by 0x48DC8E2: frr_run (libfrr.c:1222)
==1125==    by 0x15E12A: main (pim_main.c:176)
==1125==  Address 0x6274d28 is 1,192 bytes inside a block of size 1,752 free'd
==1125==    at 0x48369AB: free (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==1125==    by 0x174FF1: pim_vrf_delete (pim_instance.c:181)
==1125==    by 0x4925480: vrf_delete (vrf.c:264)
==1125==    by 0x4925480: vrf_delete (vrf.c:238)
==1125==    by 0x49332C7: zclient_vrf_delete (zclient.c:2187)
==1125==    by 0x4934319: zclient_read (zclient.c:4003)
==1125==    by 0x492249C: thread_call (thread.c:2008)
==1125==    by 0x48DC8D7: frr_run (libfrr.c:1223)
==1125==    by 0x15E12A: main (pim_main.c:176)
==1125==  Block was alloc'd at
==1125==    at 0x4837B65: calloc (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==1125==    by 0x48E80AF: qcalloc (memory.c:116)
==1125==    by 0x1750DA: pim_instance_init (pim_instance.c:90)
==1125==    by 0x1750DA: pim_vrf_new (pim_instance.c:161)
==1125==    by 0x4924FDC: vrf_get (vrf.c:183)
==1125==    by 0x493334C: zclient_vrf_add (zclient.c:2157)
==1125==    by 0x4934319: zclient_read (zclient.c:4003)
==1125==    by 0x492249C: thread_call (thread.c:2008)
==1125==    by 0x48DC8D7: frr_run (libfrr.c:1223)
==1125==    by 0x15E12A: main (pim_main.c:176)

and you do this series of events:

a) Create a vrf, put an interface in it
b) Turn on pim on that interface and turn on pim in that vrf
c) Delete the vrf
d) Do anything with the interface, in this case shutdown the system

The move of the interface to a new vrf is leaving the pim_ifp->pim pointer pointing
at the old pim instance, which was just deleted, so the instance pointer was freed.

Let's clean up the pim pointer in the interface pointer as well.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
(cherry picked from commit e60308f498c356227df5dbc6cc04015a3b213f1d)