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")
diff --git a/src/stats.c b/src/stats.c
index 791b792..2fdaaef 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -4458,7 +4458,7 @@
 	}
 
 	if (appctx->st0 == STAT_HTTP_DUMP) {
-		trash_chunk = b_make(trash.area, trash.size, 0, 0);
+		trash_chunk = b_make(trash.area, res->buf.size, 0, 0);
 		/* adjust buffer size to take htx overhead into account,
 		 * make sure to perform this call on an empty buffer
 		 */