BUG/MINOR: hlua/htx: Respect the reserve when HTX data are sent

The previous commit 7e145b3e2 ("BUG/MINOR: hlua: Don't use
channel_htx_recv_max()") is buggy. The buffer's reserve must be respected.

This patch must be backported to 2.0 and 1.9.

(cherry picked from commit 9060fc02b50db59ea97abedc2fce81cbf4eebcfe)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/hlua.c b/src/hlua.c
index 592a0ab..af24004 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -4555,7 +4555,7 @@
 	int l = MAY_LJMP(luaL_checkinteger(L, 3));
 	int max;
 
-	max = htx_free_data_space(htx);
+	max = htx_get_max_blksz(htx, channel_htx_recv_max(res, htx));
 	if (!max)
 		goto snd_yield;