sched: prevent out of bound access in calc_sg_energy

Half-baked change was ported earlier with 'commit 575464db
("sched: prevent out of bound access in sched_group_energy()")'.
Complete the change by bringing in missed changes, update
return value of the function and checks as necessary.

Change-Id: If9796c829c091e461231569dc38c5e5456f58037
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>