acpi: check for pxm_to_node_map overflow

It is hardly (if ever) possible but in case of broken _PXM entry we could
reach out of pxm_to_node_map array bounds in acpi_map_pxm_to_node() call.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>