sched: Set curr/prev_window_cpu pointers to NULL in sched_exit()

trace_sched_update_task_ravg relies on NULL pointers to ensure that
it doesn't access them. Make sure that when a task exits, these
pointers are set to NULL. Otherwise any call to update_task_ravg()
between sched_exit() and releasing the task structure will access
bogus pointers. In some cases those memory locations are unmapped
and cause a kernel panic.

Change-Id: I9eebb4fb35aca2c8424bfb29ae9d833650dc5ad4
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>