avformat/utils: use the existing packet reference when parsing complete frames

If the parser returns full frames, then the output pointer retured by
av_parser_parse2() is guaranteed to point to data contained in the
input packet's buffer.

Create a new reference to said buffer in that case, to avoid
unnecessary data copy when queueing the packet later in the function.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>