sched/walt: Fix kernel panic issue by uninitialized data

Threre is possible that time_delta is not initialized and caused
'Division by zero' exception, so move this trace into if condition
so that uninitialized data will not be used.

Change-Id: If7eb8df1c2cd7fe958b8a672d1f3b9c117caeed6
Signed-off-by: Biao long <blong@codeaurora.org>