migration/rdma: Fix qemu_rdma_cleanup null check

If the migration fails before the channel is open (e.g. a bad
address) we end up in the cleanup with rdma->channel==NULL.

Spotted by Coverity: CID 1398634
Fixes: fbbaacabSigned-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190214185351.5927-1-dgilbert@redhat.com>
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NPeter Xu <peterx@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>

migration/rdma.c

@@ -2321,7 +2321,9 @@ static void qemu_rdma_cleanup(RDMAContext *rdma)
         rdma->connected = false;
     }
 
-    qemu_set_fd_handler(rdma->channel->fd, NULL, NULL, NULL);
+    if (rdma->channel) {
+        qemu_set_fd_handler(rdma->channel->fd, NULL, NULL, NULL);
+    }
     g_free(rdma->dest_blocks);
     rdma->dest_blocks = NULL;