tipc: fix a double kfree_skb()

tipc_udp_xmit() drops the packet on error, there is no
need to drop it again.

Fixes: ef20cd4d ("tipc: introduce UDP replicast")
Reported-and-tested-by:  <syzbot+eae585ba2cc2752d3704@syzkaller.appspotmail.com>
Cc: Ying Xue <ying.xue@windriver.com>
Cc: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>