[BUG] fix harmless but wrong fd insertion sequence

In backend.c, we had an EV_FD_SET() called before fd_insert().
This is wrong because fd_insert updates maxfd which might be
used by some of the pollers during EV_FD_SET(), although this
is not currently the case.
diff --git a/src/backend.c b/src/backend.c
index f9b6dd1..2a39d77 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -1815,9 +1815,9 @@
 	fdtab[fd].peeraddr = (struct sockaddr *)&s->srv_addr;
 	fdtab[fd].peerlen = sizeof(s->srv_addr);
 
-	EV_FD_SET(fd, DIR_WR);  /* for connect status */
-    
 	fd_insert(fd);
+	EV_FD_SET(fd, DIR_WR);  /* for connect status */
+
 	if (s->srv) {
 		s->srv->cur_sess++;
 		if (s->srv->cur_sess > s->srv->cur_sess_max)