avformat/hls: Use av_init_pkt() directly

and remove reset_packet(). The packet's data pointer is already zeroed,
so the only thing that reset_packet() does that av_init_pkt() doesn't is
redundant.

Reviewed-by: Steven Liu <lingjiujianke@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
