BUG/MINOR: stats: fix source buffer size for http dump

BUG/MINOR: stats: fix source buffer size for http dump

In ("BUG/MINOR: stats: use proper buffer size for http dump"),
we used trash.size as source buffer size before applying the htx
overhead computation.

It is safer to use res->buf.size instead since res_htx (which is <htx> argument
passed to stats_putchk() in http context) is made from res->buf:

in http_stats_io_handler:
    | res_htx = htx_from_buf(&res->buf);

This will prevent the hang bug from showing up again if res->buf.size were to be
less than trash.size (which is set according to tune.bufsize).

This should be backported with ("BUG/MINOR: stats: use proper buffer size for http dump")