When the gotten object is NULL (probably due to ENOMEM), kmemleak_erase() is
unnecessary here, It just sets NULL to where already is NULL.  Add a condition.
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NJ. R. Okajima <hooanon05@yahoo.co.jp>
Signed-off-by: NPekka Enberg <penberg@cs.helsinki.fi>