x86/microcode/intel: Fix out of bounds memory access to the extended header

Improper pointer arithmetics when calculating the address of the
extended header could lead to an out of bounds memory read and kernel
panic.

Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Link: http://lkml.kernel.org/r/20150225094125.GB30434@chrystal.uk.oracle.com


Signed-off-by: Borislav Petkov <bp@suse.de>