[FIX] res_users: fixed on_change_login that crashed
when changing the login by a void value.
[FIX] res_users: change password wizard: fixed crash when submitting the form.
Indeed unlinking wizard content before actually closing it seems to
cause a crash. Instead of unlinking lines with user - password data
we now write a void value on the new password. The stored content
does not contain valuable data after submitting the form.