net: ipv6: Clear pending buf if NS cannot be sent

If for some reason the neighbor solicitation cannot be sent,
then we need to unref the pending packet as that would never
be freed otherwise.

Change-Id: Ied12a9b3a027a2aad3fafd0b5b36c56b5f042c28
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>