avpacket: Replace av_free_packet with av_packet_unref

`av_packet_unref` matches the AVFrame ref-counted API and can be used as
a drop in replacement.

Deprecate `av_free_packet`.

libavformat/rtpdec_xiph.c

@@ -141,7 +141,7 @@ static int xiph_handle_packet(AVFormatContext *ctx, PayloadContext *data,
                 data->split_buf = av_malloc(data->split_buf_size);
                 if (!data->split_buf) {
                     av_log(ctx, AV_LOG_ERROR, "Out of memory.\n");
-                    av_free_packet(pkt);
+                    av_packet_unref(pkt);
                     return AVERROR(ENOMEM);
                 }
             }