BUILD: conn_stream: avoid null-deref warnings on gcc 6

gcc 6 continues its saga with excessive reports of null-deref warnings.
This time it was in the IS_HTX_CS() macro. Let's use __cs_conn() after
cs_conn() was checked.
diff --git a/include/haproxy/conn_stream.h b/include/haproxy/conn_stream.h
index fffbc62..e69f9c1 100644
--- a/include/haproxy/conn_stream.h
+++ b/include/haproxy/conn_stream.h
@@ -32,7 +32,7 @@
 struct stream_interface;
 struct check;
 
-#define IS_HTX_CS(cs)     (cs_conn(cs) && IS_HTX_CONN(cs_conn(cs)))
+#define IS_HTX_CS(cs)     (cs_conn(cs) && IS_HTX_CONN(__cs_conn(cs)))
 
 struct conn_stream *cs_new();
 void cs_free(struct conn_stream *cs);