msg/async/rdma: cosmetics initialize ibv_send_wr* var

msg/async/rdma: cosmetics initialize ibv_send_wr* var

API usage:
int ibv_post_send(struct ibv_qp *qp, struct ibv_send_wr *wr, struct ibv_send_wr **bad_wr)
Input Parameters:
   qp struct ibv_qp from ibv_create_qp
   wr first work request (WR)
Output Parameters:
   bad_wr pointer to first rejected WR
Return Value:
   0 on success, -1 on error.
   If the call fails, errno will be set to indicate the reason for the failure.
To avoid wrong checking return value, it's better to initialize the
value to be nullptr.

Signed-off-by: Changcheng Liu <changcheng.liu@aliyun.com>