sched/fair: fix possible usage of uninitialized variables

It is possible that uninitialized variables would be used
when sched_task_util trace point is enabled in some cases.
Fix it by using local variables.

Change-Id: Ib8edf0b40fa735ff98907ed2f29b9f1b4ec871cd
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>