rpc: fix segmentation fault caused by null client-sock

The client-sock could have been set to NULL by eventloop thread
after async event fired.

src/rpc/virnetclient.c

@@ -644,6 +644,7 @@ virNetClientMarkClose(virNetClientPtr client,
                       int reason)
 {
     VIR_DEBUG("client=%p, reason=%d", client, reason);
+    if (client->sock)
         virNetSocketRemoveIOCallback(client->sock);
     client->wantClose = true;
     client->closeReason = reason;