Clear the nativeRequestContext on dispose

Author: davidfowl

src/Servers/HttpSys/src/AsyncAcceptContext.cs

@@ -198,6 +198,7 @@ protected virtual void Dispose(bool disposing)
                 {
                     _nativeRequestContext.ReleasePins();
                     _nativeRequestContext.Dispose();
+                    _nativeRequestContext = null;
                 }
             }
         }