RDMA/iw_cxgb4: Fix the unchecked ep dereference

The patch 944661dd: "RDMA/iw_cxgb4: atomically lookup ep and get a
reference" from May 6, 2016, leads to the following Smatch complaint:

    drivers/infiniband/hw/cxgb4/cm.c:2953 terminate()
    error: we previously assumed 'ep' could be null (see line 2945)

Fixes: 944661dd ("RDMA/iw_cxgb4: atomically lookup ep and get a reference")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>