avformat/utils: Remove redundant save+restore

avformat/utils: Remove redundant save+restore

If the size of the input packet is zero, av_grow_packet() used to call
av_new_packet() which would initialize the packet and (in particular)
reset the pos field. This behaviour (which was never documented and
arguably always contradicted the documented behaviour) was changed in
2fe04630. This means that it is unnecessary to save and restore the
packet's position in append_packet_chunked().

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>