Make max_total_cltv_expiry_delta include the final CLTV

This fixes an integer underflow found by the `router` fuzz target
in CI.

Author: TheBlueMatt

lightning/src/routing/router.rs

@@ -670,6 +670,9 @@ where L::Target: Logger {
 			}
 		}
 	}
+	if payment_params.max_total_cltv_expiry_delta <= final_cltv_expiry_delta {
+		return Err(LightningError{err: "Can't find a route where the total CLTV expiry delta is below the final CLTV expiry.".to_owned(), action: ErrorAction::IgnoreError});
+	}
 
 	// The general routing idea is the following:
 	// 1. Fill first/last hops communicated by the caller.
@@ -866,9 +869,9 @@ where L::Target: Logger {
 					// In order to already account for some of the privacy enhancing random CLTV
 					// expiry delta offset we add on top later, we subtract a rough estimate
 					// (2*MEDIAN_HOP_CLTV_EXPIRY_DELTA) here.
-					let max_total_cltv_expiry_delta = payment_params.max_total_cltv_expiry_delta
+					let max_total_cltv_expiry_delta = (payment_params.max_total_cltv_expiry_delta - final_cltv_expiry_delta)
 						.checked_sub(2*MEDIAN_HOP_CLTV_EXPIRY_DELTA)
-						.unwrap_or(payment_params.max_total_cltv_expiry_delta);
+						.unwrap_or(payment_params.max_total_cltv_expiry_delta - final_cltv_expiry_delta);
 					let hop_total_cltv_delta = ($next_hops_cltv_delta as u32)
 						.checked_add($candidate.cltv_expiry_delta())
 						.unwrap_or(u32::max_value());