[ntcore] Fix crash on disconnect (#5788)

The object was being destroyed due to the error handling path.
Instead, defer to the next loop cycle using a one-shot timer.

Properly handle error return values from Send functions.

Fix UB in accessing one past the end of a vector.

ntcore/src/main/native/cpp/NetworkClient.cpp

@@ -242,7 +242,11 @@ void NetworkClient3::TcpConnected(uv::Tcp& tcp) {
         tcp.error.connect([this, &tcp](uv::Error err) {
           DEBUG3("NT3 TCP error {}", err.str());
           if (!tcp.IsLoopClosing()) {
-            DoDisconnect(err.str());
+            // we could be in the middle of sending data, so defer disconnect
+            uv::Timer::SingleShot(m_loop, uv::Timer::Time{0},
+                                  [this, reason = std::string{err.str()}] {
+                                    DoDisconnect(reason);
+                                  });
           }
         });
         tcp.end.connect([this, &tcp] {
@@ -412,7 +416,10 @@ void NetworkClient::WsConnected(wpi::WebSocket& ws, uv::Tcp& tcp,
   m_clientImpl->SendInitial();
   ws.closed.connect([this, &ws](uint16_t, std::string_view reason) {
     if (!ws.GetStream().IsLoopClosing()) {
-      DoDisconnect(reason);
+      // we could be in the middle of sending data, so defer disconnect
+      uv::Timer::SingleShot(
+          m_loop, uv::Timer::Time{0},
+          [this, reason = std::string{reason}] { DoDisconnect(reason); });
     }
   });
   ws.text.connect([this](std::string_view data, bool) {

ntcore/src/main/native/cpp/net/NetworkOutgoingQueue.h

@@ -280,9 +280,15 @@ void NetworkOutgoingQueue<MessageType>::SendOutgoing(uint64_t curTimeMs,
         });
       }
     }
+    if (unsent < 0) {
+      return;  // error
+    }
     if (unsent == 0) {
       // finish writing any partial buffers
       unsent = m_wire.Flush();
+      if (unsent < 0) {
+        return;  // error
+      }
     }
     int delta = it - msgs.begin() - unsent;
     for (auto&& msg : std::span{msgs}.subspan(0, delta)) {

ntcore/src/main/native/cpp/net/ServerImpl.cpp

@@ -524,6 +524,9 @@ void ServerImpl::ClientData4::SendAnnounce(TopicData* topic,
       WireEncodeAnnounce(os, topic->name, topic->id, topic->typeStr,
                          topic->properties, pubuid);
     });
+    if (unsent < 0) {
+      return;  // error
+    }
     if (unsent == 0 && m_wire.Flush() == 0) {
       return;
     }
@@ -544,6 +547,9 @@ void ServerImpl::ClientData4::SendUnannounce(TopicData* topic) {
   if (m_local) {
     int unsent = m_wire.WriteText(
         [&](auto& os) { WireEncodeUnannounce(os, topic->name, topic->id); });
+    if (unsent < 0) {
+      return;  // error
+    }
     if (unsent == 0 && m_wire.Flush() == 0) {
       return;
     }
@@ -565,6 +571,9 @@ void ServerImpl::ClientData4::SendPropertiesUpdate(TopicData* topic,
     int unsent = m_wire.WriteText([&](auto& os) {
       WireEncodePropertiesUpdate(os, topic->name, update, ack);
     });
+    if (unsent < 0) {
+      return;  // error
+    }
     if (unsent == 0 && m_wire.Flush() == 0) {
       return;
     }

ntcore/src/main/native/cpp/net/WebSocketConnection.cpp

@@ -186,7 +186,8 @@ int WebSocketConnection::Flush() {
   m_ws_frames.reserve(m_frames.size());
   for (auto&& frame : m_frames) {
     m_ws_frames.emplace_back(
-        frame.opcode, std::span{&m_bufs[frame.start], &m_bufs[frame.end]});
+        frame.opcode,
+        std::span{m_bufs}.subspan(frame.start, frame.end - frame.start));
   }
 
   auto unsentFrames = m_ws.TrySendFrames(