[PATCH] x86_64: Proper null pointer check in powernow_k8_get

This prevents crashes on dual core system when enough ticks are lost.

Replaces earlier patch by me.

Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>