ospf6d: Prevent crash in adj_ok

ospf6d: Prevent crash in adj_ok

The adj_ok thread event is being added but not killed
when the underlying interface is deleted.  I am seeing
this crash:

OSPF6: Received signal 11 at 1636142186 (si_addr 0x0, PC 0x561d7fc42285); aborting...
OSPF6: zlog_signal+0x18c                  7f227e93519a     7ffdae024590 /lib/libfrr.so.0 (mapped at 0x7f227e884000)
OSPF6: core_handler+0xe3                  7f227e97305e     7ffdae0246b0 /lib/libfrr.so.0 (mapped at 0x7f227e884000)
OSPF6: funlockfile+0x50                   7f227e863140     7ffdae024800 /lib/x86_64-linux-gnu/libpthread.so.0 (mapped at 0x7f227e84f000)
OSPF6:     ---- signal ----
OSPF6: need_adjacency+0x10                561d7fc42285     7ffdae024db0 /usr/lib/frr/ospf6d (mapped at 0x561d7fbc6000)
OSPF6: adj_ok+0x180                       561d7fc42f0b     7ffdae024dc0 /usr/lib/frr/ospf6d (mapped at 0x561d7fbc6000)
OSPF6: thread_call+0xc2                   7f227e989e32     7ffdae024e00 /lib/libfrr.so.0 (mapped at 0x7f227e884000)
OSPF6: frr_run+0x217                      7f227e92a7f3     7ffdae024ec0 /lib/libfrr.so.0 (mapped at 0x7f227e884000)
OSPF6: main+0xf3                          561d7fc0f573     7ffdae024fd0 /usr/lib/frr/ospf6d (mapped at 0x561d7fbc6000)
OSPF6: __libc_start_main+0xea             7f227e6b0d0a     7ffdae025010 /lib/x86_64-linux-gnu/libc.so.6 (mapped at 0x7f227e68a000)
OSPF6: _start+0x2a                        561d7fc0f06a     7ffdae0250e0 /usr/lib/frr/ospf6d (mapped at 0x561d7fbc6000)
OSPF6: in thread adj_ok scheduled from ospf6d/ospf6_interface.c:678 dr_election()

The crash is in the on->ospf6_if pointer is NULL.  The only way this could
happen from what I can tell is that the event is added to the system
and then we immediately delete the interface, removing the memory
but not freeing up the adj_ok thread event.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>