Check menu item still exists before editing it in multiple_email_menu_alter

If something removes the menu entry for multiple email before multiple email runs, it fatal errors due to + NULL. (Encountered this issue when testing an upgrade where the weight of a modules done via an update operation hadn't been run; my module would have run after :-/!).

Patch against an older version, but the area should still be the same, but line numbers might be off.