tty: Only store the stdin fd when it corresponds to a tty

Doing otherwise can lead to problems when using inherit-fd on stdin as
it gets closed.

Signed-off-by: Nicolas Viennot <Nicolas.Viennot@twosigma.com>

criu/tty.c

@@ -2342,13 +2342,14 @@ int tty_prep_fds(void)
 
 	if (!isatty(STDIN_FILENO))
 		pr_info("Standard stream is not a terminal, may fail later\n");
-	else
+	else {
 		stdin_isatty = true;
 
-	self_stdin_fdid = fdstore_add(STDIN_FILENO);
-	if (self_stdin_fdid < 0) {
-		pr_err("Can't place stdin fd to fdstore\n");
-		return -1;
+		self_stdin_fdid = fdstore_add(STDIN_FILENO);
+		if (self_stdin_fdid < 0) {
+			pr_err("Can't place stdin fd to fdstore\n");
+			return -1;
+		}
 	}
 
 	return 0;