Don't generate a commitment if we cannot afford a holding cell feerate

While an `update_fee` is in the holding cell, it is possible though
unlikely for HTLCs to get added to the commitment transaction such that
when we release the holding cell, we can no longer afford this new
feerate.

In that case, we previously would drop the fee update, but still send a
commitment (at the old feerate), which is a break of the specification.

We now stop generating this lonely commitment when the fee update gets
dropped upon release from the holding cell.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -6275,14 +6275,11 @@ impl<SP: Deref> FundedChannel<SP> where
 					}
 				}
 			}
-			if update_add_count == 0 && update_fulfill_count == 0 && update_fail_count == 0 && self.context.holding_cell_update_fee.is_none() {
+			let update_fee = self.context.holding_cell_update_fee.take().and_then(|feerate| self.send_update_fee(feerate, false, fee_estimator, logger));
+
+			if update_add_count == 0 && update_fulfill_count == 0 && update_fail_count == 0 && update_fee.is_none() {
 				return (None, htlcs_to_fail);
 			}
-			let update_fee = if let Some(feerate) = self.context.holding_cell_update_fee.take() {
-				self.send_update_fee(feerate, false, fee_estimator, logger)
-			} else {
-				None
-			};
 
 			let mut additional_update = self.build_commitment_no_status_check(logger);
 			// build_commitment_no_status_check and get_update_fulfill_htlc may bump latest_monitor_id