iw_cxgb4: check return value of alloc_skb

Function alloc_skb() will return a NULL pointer when there is no enough
memory. However, the return value of alloc_skb() is directly used
without validation in function send_fw_pass_open_req(). This patches
checks the return value of alloc_skb() against NULL.
Signed-off-by: NPan Bian <bianpan2016@163.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>

drivers/infiniband/hw/cxgb4/cm.c

@@ -3799,6 +3799,8 @@ static void send_fw_pass_open_req(struct c4iw_dev *dev, struct sk_buff *skb,
 	int ret;
 
 	req_skb = alloc_skb(sizeof(struct fw_ofld_connection_wr), GFP_KERNEL);
+	if (!req_skb)
+		return;
 	req = (struct fw_ofld_connection_wr *)__skb_put(req_skb, sizeof(*req));
 	memset(req, 0, sizeof(*req));
 	req->op_compl = htonl(WR_OP_V(FW_OFLD_CONNECTION_WR) | FW_WR_COMPL_F);