sched/fair: Check whether any CPUs are available

There is a possibility that there are no allowed CPUs online when we try
to select the best cpu for a small task. Add a check to ensure we don't
continue if there are no CPUs available.

CRs-fixed: 692505
Change-Id: Iff955fb0d0b07e758a893539f7bc8ea8aa09d9c4
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>