The current approach has two different calls to av_bsf_send_packet():
A normal one, sending a packet; and an extraordinary one just for
flushing. These can be unified into one by making use of the newly
documented fact that av_bsf_send_packet() allows to signal flushing via
empty packets (i.e. packets without data and side-data).

This also fixes CID 1455685 which resulted from the fact that the call
for flushing was not checked given that it couldn't fail.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>