According to the BSF API, when a BSF is finished with an input packet,
it should return AVERROR(EAGAIN) to signal that another packet should be
sent to the BSF via av_bsf_send_packet that the actual BSF can receive
via ff_bsf_get_packet[_ref]. filter_units on the other hand simply called
ff_bsf_get_packet again if the first packet received didn't result in
any output. This call of course returned AVERROR(EAGAIN) which was
returned, but it is nevertheless better to not include a fake loop.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>