Merge pull request #2545 from ermshiperete/Xamarin-24974

[corlib] Fix exception in __ComObject finalizer during shutdown

* keep a reference to a __ComObject's proxy so it doesn't get garbage
  collected
* don't create __ComObjects unnecessarily
* Fix for bug [24974](https://bugzilla.xamarin.com/show_bug.cgi?id=24974)

diff --cc mcs/class/corlib/System/__ComObject.cs
Simple merge