Add check to validate amount_msats vs invoice_request

When an invoice with an amount_msats is different from the
invoice_request's amount_msats it's rejected as invalid.

Author: slanesuke

lightning/src/offers/invoice.rs

@@ -1374,6 +1374,13 @@ impl TryFrom<PartialInvoiceTlvStream> for InvoiceContents {
 			let invoice_request = InvoiceRequestContents::try_from(
 				(payer_tlv_stream, offer_tlv_stream, invoice_request_tlv_stream)
 			)?;
+
+			if let Some(requested_amount_msats) = invoice_request.amount_msats() {
+				if amount_msats != requested_amount_msats {
+					return Err(Bolt12SemanticError::UnexpectedAmount);
+				}
+			}
+
 			Ok(InvoiceContents::ForOffer { invoice_request, fields })
 		}
 	}