This brings a performance improvement when demuxing files, most of the
improvement comes from buffer pooling unbound packets.

time ffprobe -i samples/ffmpeg-bugs/trac/ticket6132/Samsung_HDR_-_Chasing_the_Light.ts -show_packets >/dev/null 2>&1

Before:
    real    0m1.967s
    user    0m1.471s
    sys     0m0.493s

After:
    real    0m1.497s
    user    0m1.364s
    sys     0m0.129s

Based on a patch of James Almer.
Signed-off-by: Marton Balint <cus@passwd.hu>