sched: prevent out of bound access in sched_group_energy()

group_idle_state() can return INT_MAX + 1 which is undefined behaviour
when there is no CPUs in sched_group.  Prevent such by error correctly.

Change-Id: I236a90b9725edc2f394ade6533368f5d4b1a21d2
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>