avformat/utils: Improve parsing packets

avformat/utils: Improve parsing packets

Up until now, parse_packet() used a stack packet in case the stream is
flushed. But using such a packet is unnecessary as there is an AVPacket
readily available, it just needs to be used. Whether flushing is intended
or not will now be signalled by an explicit parameter rather than by
whether the packet parameter is NULL. This removes a few checks in
parse_packet(), gets rid of the initialization of the stack packet and
also reduces usage of sizeof(AVPacket) in libavformat.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>