winrt: Avoid crash on device discovery when device has to be paired

If a device has to be paired during device discovery on winrt it is
possible that the pairing dialog is shown, while discovery hits its
timeout. We have to protect against a late callback which tries to access
a deleted object.

Change-Id: I9756f993b4a31080b1ef518953bee6e75a24e315
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>