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

test/unsendable.test.ts

@@ -345,6 +345,30 @@ describe('a body the PDU\'s own data_coding cannot carry', () => {
 	});
 });
 
+describe('an address the field cannot carry', () => {
+	test('refuses it through sendSms(), with nothing reaching the socket', async t => {
+		const smsc = await dummySmsc(t);
+		const session = await bindToSmsc(t, smsc.port, { reconnect: false });
+
+		const refusals: [string, RegExp][] = [
+			['46701113311\u0000EVIL', /U\+0000 at index 11/],
+			['Kaffe一', /U\+4E00 at index 5/],
+			['😀', /U\+1F600 at index 0/],
+		];
+
+		for (const [sender, names] of refusals) {
+			const sent = await session.sendSms({ from: sender, message: 'Hello world', to });
+
+			assert.ok(sent.err instanceof Error, sender);
+			assert.match(sent.err.message, /source_addr/);
+			assert.match(sent.err.message, names);
+			assert.deepEqual(sent.smsIds, [], sender);
+		}
+
+		assert.deepEqual(smsc.octets, [], 'an address the field cannot carry never reaches the socket');
+	});
+});
+
 describe('a time no peer can read', () => {
 	const invalid = new Date('nope');