The APIC ID returned by hard_smp_processor_id can be beyond
NR_CPUS and then overflow the x86_cpu_to_apic[] array.

Add a check for overflow. If it happens then the slow loop below
will catch.

Bug pointed out by Doug Thompson
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>