winrt: Camera stop while probing frames leads to crash on certain Lumia devices

Steps:
1 . Run attached example
2. Toggle camera on/off repeatedly
Expected: Camera turns on and off
Actual: Some point app crashes.

System out:

D3D11: Removing Device.
Failed to lock camera frame buffer (The GPU device instance has been suspended. Use GetDeviceRemovedReason to determine the appropriate action.)
Failed to unlock camera frame buffer ({Page Unlocked}
The page protection of a locked page was changed to 'No Access' and the page was unlocked from memory and from the process.)

Frame buffers gets page locked when camera is stopped on some Lumia devices. If this happens same time application has frame mapped, unmapping will fail and suspend camera device.

Workaround is to delay camera stop until frame has been unmapped on application side.

Crash happens because loaded state handler is called twice and on second pass d->mediasink is null.