sudoers_debug_register() was not setting the active debug instance

sudoers_debug_register() was not setting the active debug instance
to sudoers_debug_instance when called from the I/O log plugin.  This
is because it relied on sudo_debug_register to do that but
sudoers_debug_parse_flags() doesn't set debug_files[]
sudoers_debug_instance is already set (we can only init sudoers
debug once).

To work around this, just make sudoers_debug_instance the active
debug instance in sudoers_debug_register() when it is already set.