generate exception in case websocket is closed so stream does not return array of 255

Frameworks/MQTTnet.NetStandard/Implementations/WebSocketStream.cs

@@ -3,6 +3,7 @@ using System.IO;
 using System.Net.WebSockets;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Core.Exceptions;
 
 namespace MQTTnet.Implementations
 {
@@ -49,6 +50,11 @@ namespace MQTTnet.Implementations
                 }
             }
 
+            if (_webSocket.State == WebSocketState.Closed)
+            {
+                throw new MqttCommunicationException( "connection closed" );
+            }
+
             return currentOffset - offset;
         }
 

Frameworks/MQTTnet.UniversalWindows/Implementations/WebSocketStream.cs

@@ -3,6 +3,7 @@ using System.IO;
 using System.Net.WebSockets;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Core.Exceptions;
 
 namespace MQTTnet.Implementations
 {
@@ -35,6 +36,11 @@ namespace MQTTnet.Implementations
                 }
             }
 
+            if (_webSocket.State == WebSocketState.Closed)
+            {
+                throw new MqttCommunicationException("connection closed");
+            }
+
             return currentOffset - offset;
         }