`BitcoinRegtestController::get_utxos(..)` stalls when providing burn height greater than tip

[fdefelici]

Considering a burnchain booted to height 101, and then asking for utxos after this height make the get_utxos stalls in an infinite loop

        let btc_controller = BitcoinRegtestController::new(..);
        btc_controller.bootstrap_chain(101);
       
        let future_block_height = 1000;
        let utxos = btc_controller.get_utxos(.., .., .., .., future_block_height);