Bugfix for ePN uninitialized use of variable

Since the help output shows a type 3 check without a provided warning or error user level,

        Example:
            -T 'Chassis Intrusion'

and the code ignores any provided values after this, my patch resets the variables warn and crit to empty (but defined!).  This makes the performance status line consistent even if they are provided (as they are ignored), and fixes a Nagios embedded Perl interpreter (ePN) error when the variable was referenced but not initialized:

            $status = $status . "$monitor_name=$monitor_reading;$warn;$crit;;";