ipv6: properly check return value in inet6_dump_all()

Make sure we call fib6_dump_end() if it happens that skb->len
is zero. rtnl_dump_all() can reset cb->args on the next loop
iteration there.

Fixes: 08e814c9 ("net/ipv6: Bail early if user only wants cloned entries")
Fixes: ae677bbb ("net: Don't return invalid table id error when dumping all families")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>