RDMA/cxgb4: Move dereference below NULL test

spatch with a semantic match is used to found this.
(http://coccinelle.lip6.fr/

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>