ensure we dispose on all exit paths

Ix.NET/Source/System.Interactive.Async/Join.cs

@@ -162,10 +162,12 @@ namespace System.Linq
                                     goto case Mode.DoLoop;
                                 }
 
-                                Dispose();
+                             
                                 break;
                         }
 
+                        Dispose();
+
                         break;
                 }