Fixes the following sparse warnings:

arch/x86_64/kernel/mce_amd.c:321:29: warning: Using plain integer as NULL pointer
arch/x86_64/kernel/mce_amd.c:410:41: warning: Using plain integer as NULL pointer
Signed-off-by: NLuiz Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>