net: ipv6: Do not try to unref null pointer

If the DAD timeouts, then the pending pointer will be null
when we remove the neighbor. Fortunately this only prints
this error message and does not cause any issues in the code.

[net/nbuf] [DBG] net_nbuf_unref_debug: (0x00118350): *** ERROR *** \
  buf 0x00000000 (nbr_clear_ns_pending():175)

Change-Id: I3e11d4aa1d90f205df591b5d5cdcf2ee7bde6c01
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>