Fix "bad wxCheckListBox index" warning when trying to create AR Code

Fixes issue [7060](https://code.google.com/p/dolphin-emu/issues/detail?id=7060)

Since the "Create AR Code" dialog adds an AR code to arCodes but doesn't add a checkbox to the cheat list, an error occurs during saving when trying to access the checkbox associated with the added code. To fix this, the save method now checks the index before it tries to access the checkbox.