Merged requested to merge michal/silence-a-perl-warning-output-by-stop.pl-v9_14 into v9_14
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().
(cherry picked from commit 91e5a99b)