Don't release WakeLock w/o active client

Once we see the WakeLock isn't held, we should return before attempting
to release a non-existent WakeLock.

Bug: 168002974
Test: manual
Change-Id: I0172c637d7f6f22888c126da42d4fd1fe45fe0db
(cherry picked from commit 386c3fcc8f6678d330f72e900c40f890db9b76f4)
1 file changed