logging/vty: fix: do not close stderr in vty_close()

Since Icdeaea67a06da3a2f07b252e455629559ecc1829, we use stderr for
printing warnings while parsing the VTY configuration files. Make
sure we do not close() stderr. Otherwise stderr logging gets broken.

Change-Id: I6ecc85555d102f5911d50ed5ac54933c766fa84d
Fixes: Icdeaea67a06da3a2f07b252e455629559ecc1829
diff --git a/src/vty/vty.c b/src/vty/vty.c
index 3357d5a..cd252ad 100644
--- a/src/vty/vty.c
+++ b/src/vty/vty.c
@@ -231,7 +231,7 @@
 	vector_unset(vtyvec, vty->fd);
 
 	/* Close socket. */
-	if (vty->fd > 0) {
+	if (vty->fd > 0 && vty->fd != fileno(stderr)) {
 		close(vty->fd);
 		vty->fd = -1;
 	}