Confirm user object when checking _WithoutTickets

When shredding users, check the user object before processing
_WithoutTickets. If it's not properly loaded, return to allow
FilterWithoutTickets to skip that user and continue processing.

This should only happen in strange cases, like using the
MergeUsers extension where one user record can try to load
another user record that shredder has already deleted.
Merge branch '4.0/check-user-object-in-shredder' into 4.0-trunk