Don't show tripwire code if database file doesn't exist

src/freedombone-controlpanel

@@ -1269,6 +1269,9 @@ function security_settings {
 }
 
 function show_tripwire_verification_code {
+    if [ ! -f /var/lib/tripwire/${HOSTNAME}.twd ]; then
+        return
+    fi
     clear
     echo ''
     echo $'Tripwire Verification Code'