[UWP] fixes crash after reset corner radius (#6744)

Xamarin.Forms.Platform.UAP/FormsButton.cs

@@ -71,9 +71,11 @@ namespace Xamarin.Forms.Platform.UWP
 
 		void UpdateBorderRadius()
 		{
-
 			if (_contentPresenter != null)
-				_contentPresenter.CornerRadius = new Windows.UI.Xaml.CornerRadius(BorderRadius);
+			{
+				var radius = BorderRadius == -1 ? 0 : BorderRadius;
+				_contentPresenter.CornerRadius = new Windows.UI.Xaml.CornerRadius(radius);
+			}
 		}
 	}
 }