ng: wakeup-latency script cleanup

Some minor fixes for the wakeup-latency script:

 - Fix nuisance 'use of uninitialized value' warnings

 - Avoid divide-by-zero error

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <1273466820-9330-5-git-send-email-tzanussi@gmail.com>
Signed-off-by: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>