fix bug #466878 if no folder is select to un/subscribe

src/folders_subscribe.php

@@ -32,6 +32,9 @@
          header("Location: $location/folders.php?success=unsubscribe");
       }
    }
+   if (!isset($mailbox)) {
+         header("Location: $location/folders.php");
+   }
    sqimap_logout($imapConnection);
 
    /*
@@ -53,4 +56,4 @@
    echo "</CENTER>";
    echo "</BODY></HTML>";
    */
-?>
+?>