Cover the guarded address writers, and cut what the code already says

test/pdu.test.ts

@@ -128,7 +128,6 @@ describe('parsing real PDUs', () => {
 		assert.equal(pduObj.tlvs.message_state?.tagValue, 2);
 	});
 
-	// 'ascii' masks bit 7 on the way in, so this address used to come back Kaffei.
 	test('keeps an alphanumeric sender whole through the wire and back', () => {
 		const pdu = encode({
 			cmdName: 'deliver_sm',
@@ -143,12 +142,9 @@ describe('parsing real PDUs', () => {
 
 		assert.ok(pdu.includes(Buffer.from('Kaffeé', 'latin1')));
 		assert.equal(decode(pdu).params.source_addr, 'Kaffeé');
-		assert.ok(objToPdu({ cmdName: 'deliver_sm', params: { source_addr: '一' } }).err instanceof Error);
 	});
 
-	// The peer reads source_addr to the first NULL and every mandatory field behind it shifts, so an
-	// application forwarding a customer's sender id could have a PDU rewritten under it.
-	test('refuses an address carrying its own terminator', () => {
+	test('refuses an address the field cannot carry rather than truncating it', () => {
 		const smuggled = objToPdu({
 			cmdName: 'submit_sm',
 			params: { destination_addr: '46709771337', source_addr: '46701113311\u0000EVIL' },
@@ -156,6 +152,7 @@ describe('parsing real PDUs', () => {
 
 		assert.ok(smuggled.err instanceof Error);
 		assert.equal(smuggled.buffer, undefined);
+		assert.ok(objToPdu({ cmdName: 'deliver_sm', params: { source_addr: '一' } }).err instanceof Error);
 	});
 });