Profile type validation for On Behalf profiles is checking the custom_pre profile instead

If you add a membership or contribution custom field to the Top of Page profile for a contribution page AND you have enabled contributing on behalf of an organization - you will get a spurious fatal error: 'Mixed profile not allowed for on behalf of registration/sign up.'

This is because the code which is supposed to prevent contribution and / or membership fields in the On Behalf profile was actually checking the wrong profile (CRM_Contribute_Form_Contribution_Main::preProcess).