Silence a Perl warning output by stop.pl

Merged Michał Kępień requested to merge michal/silence-a-perl-warning-output-by-stop.pl into master

On Unix systems, the CYGWIN environment variable is not set at all when BIND system tests are run. If a named instance crashes on shutdown or otherwise fails to clean up its pidfile and the CYGWIN environment variable is not set, stop.pl will print an uninitialized value warning on standard error. Prevent this by using defined().

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