[BUG] check_post: limit analysis to the buffer length

If "balance url_param XXX check_post" is used, we must bound the
number of bytes analysed to the buffer's length.
(cherry picked from commit dc8017ced6a8ec699a50a409f3c8ce5928ea70fa)

src/backend.c

@@ -1248,6 +1248,9 @@ struct server *get_server_ph_post(struct session *s)
 		len = chunk;
 	}
 
+	if (len > req->l - body)
+		len = req->l - body;
+
 	p = params;
 
 	while (len > plen) {