hw/rdma: Fix possible usage of a NULL pointer

Coverity CID 1390586; The cq handle is provided by the guest
and cannot be trusted to be previuosly allocated.
Fix it by exiting the completion flow.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Message-Id: <20180430200223.4119-3-marcel.apfelbaum@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

hw/rdma/vmw/pvrdma_qp_ops.c

@@ -216,6 +216,7 @@ void pvrdma_cq_poll(RdmaDeviceResources *dev_res, uint32_t cq_handle)
     cq = rdma_rm_get_cq(dev_res, cq_handle);
     if (!cq) {
         pr_dbg("Invalid CQ# %d\n", cq_handle);
+        return;
     }
 
     rdma_backend_poll_cq(dev_res, &cq->backend_cq);