BUG/MINOR: stats/htx: Respect the reserve when the stats page is dumped

As for the cache applet, this one must respect the reserve on HTX streams. This
patch is tagged as MINOR because it is unlikely to fully fill the channel's
buffer. Some tests are already done to not process almost full buffer.

This patch must be backported to 1.9.
diff --git a/src/stats.c b/src/stats.c
index 5e54dd5..ebd95d3 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -243,6 +243,8 @@
 static int stats_putchk(struct channel *chn, struct htx *htx, struct buffer *chk)
 {
 	if (htx) {
+		if (chk->data >= channel_htx_recv_max(chn, htx))
+			return 0;
 		if (!htx_add_data(htx, ist2(chk->area, chk->data)))
 			return 0;
 		channel_add_input(chn, chk->data);