tipc: fix return value check in tipc_mcast_send_sync()

Fix the return value check which testing the wrong variable
in tipc_mcast_send_sync().

Fixes: c55c8eda ("tipc: smooth change between replicast and broadcast")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Acked-by: NJon Maloy <jon.maloy@ericsson.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/tipc/bcast.c

@@ -329,7 +329,7 @@ static int tipc_mcast_send_sync(struct net *net, struct sk_buff *skb,
 
 	/* Allocate dummy message */
 	_skb = tipc_buf_acquire(MCAST_H_SIZE, GFP_KERNEL);
-	if (!skb)
+	if (!_skb)
 		return -ENOMEM;
 
 	/* Preparing for 'synching' header */