fix intermittent error in global destruction

In a largish application using Net::Whois::RIPE, I sometimes get

        (in cleanup) Can't call method "remove" on an undefined value at /usr/share/perl5/Net/Whois/RIPE.pm line 440 during global destruction.

I don't know what exact circumstances lead to it, but IMHO it's not worth dying for :-)

(fixed version of an earlier pull request that included quite some garbage)