net: hns3: check msg_data before memcpy in hclgevf_send_mbx_msg

The value of msg_data may be NULL in some cases, which will cause
errors reported by some compiler.

So this patch adds a check to fix it.

Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>