migration/rdma: Fix cm_event used before being initialized

migration/rdma: Fix cm_event used before being initialized

A segmentation fault was triggered when i try to abort a postcopy + rdma
migration.

since rdma_ack_cm_event releases a uninitialized cm_event in these case.

like below:
2496     ret = rdma_get_cm_event(rdma->channel, &cm_event);
2497     if (ret) {
2498         perror("rdma_get_cm_event after rdma_connect");
2499         ERROR(errp, "connecting to destination!");
2500         rdma_ack_cm_event(cm_event); <<<< cause segmentation fault
2501         goto err_rdma_source_connect;
2502     }

Refer to the rdma_get_cm_event() code, cm_event will be
updated/changed only if rdma_get_cm_event() returns 0. So it's okey to
remove the ack in error patch.

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Message-Id: <20210519064740.10828-1-lizhijian@cn.fujitsu.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>