E_NOTICE thrown when duplicating

When a menu is duplicated, metadata is duplicated as well, and the original metadata is checked to see if it is serialized. When unserialize is fed data that is not serialized, in addition to returning false, it also throws a E_NOTICE.

WordPress has a helper function called maybe_unserialize which checks to see if data is serialized first, and also squelches any potential error output from unserialize. This function should probably be used instead of the conditional that checks for a false from unserialize