x86, amd, mce: Avoid NULL pointer reference on CPU northbridge lookup

When booting on a federated multi-server system (NumaScale), the
processor Northbridge lookup returns NULL; add guards to prevent this
causing an oops.

On those systems, the northbridge is accessed through MMIO and the
"normal" northbridge enumeration in amd_nb.c doesn't work since we're
generating the northbridge ID from the initial APIC ID and the last
is not unique on those systems. Long story short, we end up without
northbridge descriptors.

Signed-off-by: Daniel J Blueman <daniel@numascale-asia.com>
Cc: stable@vger.kernel.org # 3.6
Link: http://lkml.kernel.org/r/1349073725-14093-1-git-send-email-daniel@numascale-asia.com
[ Boris: beef up commit message ]
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>