Fix async client wait

Needed for REST

DearFTP/Connection/DataConnection.cs

@@ -3,6 +3,8 @@ using System.Collections.Generic;
 using System.Net;
 using System.Net.Sockets;
 using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
 
 namespace DearFTP.Connection
 {
@@ -14,11 +16,32 @@ namespace DearFTP.Connection
         public TcpClient Client { get; private set; }
         public NetworkStream Stream { get; private set; }
         public int Port => ((IPEndPoint)Listener.LocalEndpoint).Port;
-        public bool IsAvailable { get; private set; }
+        public bool IsAvailable
+        {
+            get
+            {
+                if (_acceptTask == null)
+                {
+                    return false;
+                }
+                else if (_acceptTask.Wait(Timeout))
+                {
+                    return true;
+                }
+                else
+                {
+                    _acceptTask.Dispose();
+                    _acceptTask = null;
+                    return false;
+                }
+            }
+        }
+
+        private Task _acceptTask;
 
         public DataConnection()
         {
-            IsAvailable = false;
+
         }
 
         public void Create()
@@ -35,28 +58,15 @@ namespace DearFTP.Connection
 
         public void AcceptClient()
         {
-            var task = Listener.AcceptTcpClientAsync();
-
-            if (task.Wait(Timeout))
+            _acceptTask = Listener.AcceptTcpClientAsync().ContinueWith(t =>
             {
-                Client = task.Result;
+                Client = t.Result;
                 Stream = Client.GetStream();
-
-                IsAvailable = true;
-            }
-        }
-        public async void AcceptClientAsync()
-        {
-            Client = await Listener.AcceptTcpClientAsync();
-            Stream = Client.GetStream();
-
-            IsAvailable = true;
+            });
         }
 
         public void Close()
         {
-            IsAvailable = false;
-
             Stream.Close();
             Client.Close();
             Listener.Stop();