Avoid zero size allocation in cache_k8_northbridges()

kmalloc for flush_words resulted in zero size allocation when no
k8_northbridges existed.  Short circuit the code path for this case.

Also remove uneeded zeroing of num_k8_northbridges just after checking if
it is zero.

Signed-off-by: Ben Collins <bcollins@ubuntu.com>
Cc: Andi Kleen <ak@suse.de>
Cc: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>