x86: 0 -> NULL, for arch/x86_64

When comparing a pointer, it's clearer to compare it to NULL than to 0.

[ tglx: arch/x86 adaptation ]

Signed-off-by: Yoann Padioleau <padator@wanadoo.fr>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: ak@suse.de
Cc: discuss@x86-64.org
Cc: akpm@linux-foundation.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>