perf/trace/scripting: workqueue-stats script cleanup

Some minor fixes for the workqueue-stats script:

 - Fix nuisance 'use of uninitialized value' warnings

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <1273466820-9330-6-git-send-email-tzanussi@gmail.com>
Signed-off-by: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>