avformat/utils: Move the reference to the packet list

avformat/utils: Move the reference to the packet list

Up until now, ff_packet_list_put had a flaw: When it moved a packet to
the list (meaning, when it ought to move the reference to the packet
list instead of creating a new one via av_packet_ref), it did not reset
the original packet, confusing the ownership of the data in the packet.
This has been done because some callers of this function were not
compatible with resetting the packet.

This commit changes these callers and fixes this flaw. In order to
indicate that the ownership of the packet has moved to the packet list,
pointers to constant AVPackets are used whenever the target of the
pointer might already be owned by the packet list.

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