[PATCH] KVM: Fix oops on oom

__free_page() doesn't like a NULL argument, so check before calling it.  A
NULL can only happen if memory is exhausted during allocation of a memory
slot.

Signed-off-by: Avi Kivity <avi@qumranet.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>