libosmovty: simplify condition checking vty->fd in vty_close()

On POSIX systems, standard I/O streams - stdin, stdout, and stderr,
always have default file descriptors 0, 1, and 2 respectively.

Change-Id: Ied35d142af0ba0f5ad78975b8f22c35b32d6ff71
diff --git a/src/vty/vty.c b/src/vty/vty.c
index 8cc2a3a..ebdf9fc 100644
--- a/src/vty/vty.c
+++ b/src/vty/vty.c
@@ -231,8 +231,8 @@
 	/* Unset vector. */
 	vector_unset(vtyvec, vty->fd);
 
-	/* Close socket. */
-	if (vty->fd > 0 && vty->fd != fileno(stderr)) {
+	/* Close socket (ignore standard I/O streams). */
+	if (vty->fd > 2) {
 		close(vty->fd);
 		vty->fd = -1;
 	}