Return early if required UTXOs already big enough

If the required UTXO set is already bigger (including fees) than the
amount required for the transaction we can return early, no need to go
through the BNB algorithm or random selection.

src/wallet/coin_selection.rs

@@ -345,6 +345,14 @@ impl<D: Database> CoinSelectionAlgorithm<D> for BranchAndBoundCoinSelection {
             .try_into()
             .expect("Bitcoin amount to fit into i64");
 
+        if curr_value > actual_target {
+            return Ok(BranchAndBoundCoinSelection::calculate_cs_result(
+                vec![],
+                required_utxos,
+                fee_amount,
+            ));
+        }
+
         Ok(self
             .bnb(
                 required_utxos.clone(),