sched/core: Fix null-pointer dereference

Turning on sched_use_pelt causes a null-pointer dereference in
scheduler. Add correct checking of varable.

CRs-fixed: 1042656
Change-Id: I6e6a57e24b41e4b3d049bfcf694b9ad7e2144dd5
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>