[PATCH] x86_64: minor odering correction to dump_pagetable()

Checking of the validity of pointers should be consistently done before
dereferencing the pointer.

Signed-Off-By: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>