Fix tracking of collected value across pathfinding iterations

If we end up "paying" for an `htlc_minimum_msat` with fees, we
increment `already_collected_value_msat` by more than the amount
of the path that we collected (who's `value_contribution_msat` is
higher than the total payment amount, despite having been reduced
down to the payment amount).

This throws off our total value collection target, though in the
coming commit(s) it would also throw off our path selection
calculations.

lightning/src/routing/router.rs

@@ -1464,7 +1464,7 @@ where L::Target: Logger {
 		// Both these cases (and other cases except reaching recommended_value_msat) mean that
 		// paths_collection will be stopped because found_new_path==false.
 		// This is not necessarily a routing failure.
-		'path_construction: while let Some(RouteGraphNode { node_id, lowest_fee_to_node, total_cltv_delta, value_contribution_msat, path_htlc_minimum_msat, path_penalty_msat, path_length_to_node, .. }) = targets.pop() {
+		'path_construction: while let Some(RouteGraphNode { node_id, lowest_fee_to_node, total_cltv_delta, mut value_contribution_msat, path_htlc_minimum_msat, path_penalty_msat, path_length_to_node, .. }) = targets.pop() {
 
 			// Since we're going payee-to-payer, hitting our node as a target means we should stop
 			// traversing the graph and arrange the path out of what we found.
@@ -1530,7 +1530,9 @@ where L::Target: Logger {
 				// on some channels we already passed (assuming dest->source direction). Here, we
 				// recompute the fees again, so that if that's the case, we match the currently
 				// underpaid htlc_minimum_msat with fees.
+				debug_assert_eq!(payment_path.get_value_msat(), value_contribution_msat);
 				payment_path.update_value_and_recompute_fees(cmp::min(value_contribution_msat, final_value_msat));
+				value_contribution_msat = cmp::min(value_contribution_msat, final_value_msat);
 
 				// Since a path allows to transfer as much value as
 				// the smallest channel it has ("bottleneck"), we should recompute