lavfi: avfilter_graph_request_oldest: request a frame again before returning.

lavfi: avfilter_graph_request_oldest: request a frame again before returning.

With min_samples, if a frame arrives but is too small, it clears
frame_wanted_out. In most cases, the destination filter would be
activated again later because of frame_wanted_out on its own
outputs, but not sinks.
avfilter_graph_request_oldest() is doing the work of the sink
itself, and is therefore allowed to use frame_blocked_in.