f Assert unique manual payment hashes

Document the pending-store invariant that a manual BOLT11 hash
maps to at most one pending payment id.

Co-Authored-By: HAL 9000

Author: tnull

src/payment/pending_payment_store.rs

@@ -246,6 +246,10 @@ where
 	) -> Option<PendingPaymentDetails> {
 		let index = self.manual_bolt11_payment_hash_index.lock().expect("lock");
 		let ids = index.get(payment_hash)?;
+		debug_assert!(
+			ids.len() <= 1,
+			"manual BOLT11 payment hash maps to multiple pending payment IDs"
+		);
 		ids.iter().find_map(|id| self.inner.get(id))
 	}