nbd: fix memory leak on socket_connect failed

When TCP connection fails between nbd server and client,
the local var, sioc, memory leak.

This patch fixes the memory leak.
Signed-off-by: Nyaolujing <yaolujing@huawei.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1491005709-29989-1-git-send-email-yaolujing@huawei.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

block/nbd.c

@@ -311,6 +311,7 @@ static QIOChannelSocket *nbd_establish_connection(SocketAddress *saddr,
                                     saddr,
                                     &local_err);
     if (local_err) {
+        object_unref(OBJECT(sioc));
         error_propagate(errp, local_err);
         return NULL;
     }