ffprobe: replace av_destruct_packet() with av_free_packet()

av_destruct_packet() always frees the packet data even when the demuxer
is going to re-use it, thus causing crashes when decoding audio
frames (as implemented in a pending patch).

av_free_packet() is used instead, as it allows each demuxer to set the
right packet data releasing mechanism through the pkt->destruct callback.
