C++: Fix DataProducer::Send invalid memory access (#1190)

'subchannels' are optional and hence they need to be checked before
accessing them.

Remove test expectation that assumed we were receiving all messages in
incremental mode. TS tests failure was being masked by [1], I don't know
why Rust wasn't failing but definitely consumers are not receiving
everything sent by the producer since in the tests both are paused at
some time.

[1]: #1188

node/src/tests/test-DirectTransport.ts

@@ -211,7 +211,7 @@ test('dataProducer.send() succeeds', async () =>
 				expect(ppid).toBe(53); // PPID of WebRTC DataChannel binary.
 			}
 
-			expect(id).toBe(++lastRecvMessageId);
+			++lastRecvMessageId;
 		});
 	});
 

rust/tests/integration/direct_transport.rs

@@ -157,7 +157,6 @@ fn get_stats_succeeds() {
 }
 
 #[test]
-#[ignore]
 fn send_succeeds() {
     future::block_on(async move {
         let (_worker, _router, transport) = init().await;
@@ -226,7 +225,6 @@ fn send_succeeds() {
                 }
 
                 last_recv_message_id.fetch_add(1, Ordering::SeqCst);
-                assert_eq!(id, last_recv_message_id.load(Ordering::SeqCst));
 
                 if id == num_messages {
                     let _ = received_messages_tx.lock().take().unwrap().send(());

worker/src/RTC/DataProducer.cpp

@@ -224,11 +224,14 @@ namespace RTC
 
 				std::vector<uint16_t> subchannels;
 
-				subchannels.reserve(body->subchannels()->size());
-
-				for (const auto subchannel : *body->subchannels())
+				if (flatbuffers::IsFieldPresent(body, FBS::DataProducer::SendNotification::VT_SUBCHANNELS))
 				{
-					subchannels.emplace_back(subchannel);
+					subchannels.reserve(body->subchannels()->size());
+
+					for (const auto subchannel : *body->subchannels())
+					{
+						subchannels.emplace_back(subchannel);
+					}
 				}
 
 				std::optional<uint16_t> requiredSubchannel{ std::nullopt };