Don’t call an early-returning destructor

Don’t call an early-returning destructor

This speeds things up 2.8 times in my naïve benchmarks.

This applies to code like:

    use constant DEBUG => 1;
    DESTROY { return unless DEBUG; ... }

which gets optimised down to:

    DESTROY { return; ... }

Adding such a destructor will now have almost no impact on speed in
production.