__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: NAvi Kivity <avi@qumranet.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>