transports(base_output): fix duplicate push_frame()

[aconchillo]

Please describe the changes in your PR. If it is addressing an issue, please reference that as well.

[aconchillo]

The problem was that _sink_frame_handler() was calling push_frame() and we were also calling push_frame() from _sink_task_handler(). So we don't need to do it in sink_frame_handler(). sink_frame_handler() only performs something on a frame if there's something to do. If you want to push a frame, so push it later.

[markbackman]

That solves the issue for me!