decklinkvideosrc: Fix crash when mode is not specified
In that case, we will get "VideoInputFrameArrived" callback without "VideoInputFormatChanged"
Due to an influx of spam, we have had to impose restrictions on new accounts. Please see this wiki page for instructions on how to get full permissions. Sorry for the inconvenience.
In that case, we will get "VideoInputFrameArrived" callback without "VideoInputFormatChanged"