avviddec: Report a latency even without upstream framerate
There are cases where upstream will not provide a framerate, or it won't be fixed. But if there is latency introduced by the decoder we do want to report it.
Therefore use the framerate stored in the actual decoder, which will have a default.
Fixes hangs when playing back such streams with decodebin3 (where the multiqueue will not have been informed of that downstream latency and not grow accordingly)