net: skb_condense() can also deal with empty skbs

It seems attackers can also send UDP packets with no payload at all.

skb_condense() can still be a win in this case.

It will be possible to replace the custom code in tcp_add_backlog()
to get full benefit from skb_condense()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>