Fix null exception when connection fails to start (#215)

* Fix null exception when connection fails to start

* Update ConnectionFactory.cs

* Update ServiceHubDispatcher.cs

Author: vicancy

src/Microsoft.Azure.SignalR.AspNet/Infrastructure/ConnectionFactory.cs

@@ -75,6 +75,11 @@ public async Task<ConnectionContext> ConnectAsync(TransferFormat transferFormat,
 
         public Task DisposeAsync(ConnectionContext connection)
         {
+            if (connection == null)
+            {
+                return Task.CompletedTask;
+            }
+            
             return ((HttpConnection)connection).DisposeAsync();
         }
 

src/Microsoft.Azure.SignalR/HubHost/ServiceHubDispatcher.cs

@@ -110,6 +110,11 @@ public async Task<ConnectionContext> ConnectAsync(TransferFormat transferFormat,
 
         public Task DisposeAsync(ConnectionContext connection)
         {
+            if (connection == null)
+            {
+                return Task.CompletedTask;
+            }
+            
             return ((HttpConnection)connection).DisposeAsync();
         }