Check if tty is nil in handler

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

signals.go

@@ -109,5 +109,8 @@ func (h *signalHandler) reap() (exits []exit, err error) {
 }
 
 func (h *signalHandler) Close() error {
+	if h.tty != nil {
 		return h.tty.Close()
 	}
+	return nil
+}