Fix amount overflow in Offer parsing and building

An overflow can occur when multiplying the offer amount by the requested
quantity when checking if the given amount is enough. Return an error
instead of overflowing.

lightning/src/offers/invoice_request.rs

@@ -828,6 +828,18 @@ mod tests {
 			Ok(_) => panic!("expected error"),
 			Err(e) => assert_eq!(e, SemanticError::MissingAmount),
 		}
+
+		match OfferBuilder::new("foo".into(), recipient_pubkey())
+			.amount_msats(1000)
+			.supported_quantity(Quantity::Unbounded)
+			.build().unwrap()
+			.request_invoice(vec![1; 32], payer_pubkey()).unwrap()
+			.quantity(u64::max_value()).unwrap()
+			.build()
+		{
+			Ok(_) => panic!("expected error"),
+			Err(e) => assert_eq!(e, SemanticError::InvalidAmount),
+		}
 	}
 
 	#[test]
@@ -1123,6 +1135,23 @@ mod tests {
 				assert_eq!(e, ParseError::InvalidSemantics(SemanticError::UnsupportedCurrency));
 			},
 		}
+
+		let invoice_request = OfferBuilder::new("foo".into(), recipient_pubkey())
+			.amount_msats(1000)
+			.supported_quantity(Quantity::Unbounded)
+			.build().unwrap()
+			.request_invoice(vec![1; 32], payer_pubkey()).unwrap()
+			.quantity(u64::max_value()).unwrap()
+			.build_unchecked()
+			.sign(payer_sign).unwrap();
+
+		let mut buffer = Vec::new();
+		invoice_request.write(&mut buffer).unwrap();
+
+		match InvoiceRequest::try_from(buffer) {
+			Ok(_) => panic!("expected error"),
+			Err(e) => assert_eq!(e, ParseError::InvalidSemantics(SemanticError::InvalidAmount)),
+		}
 	}
 
 	#[test]

lightning/src/offers/offer.rs

@@ -431,7 +431,8 @@ impl OfferContents {
 		};
 
 		if !self.expects_quantity() || quantity.is_some() {
-			let expected_amount_msats = offer_amount_msats * quantity.unwrap_or(1);
+			let expected_amount_msats = offer_amount_msats.checked_mul(quantity.unwrap_or(1))
+				.ok_or(SemanticError::InvalidAmount)?;
 			let amount_msats = amount_msats.unwrap_or(expected_amount_msats);
 
 			if amount_msats < expected_amount_msats {