kmemleak: Do not return a pointer to an object that kmemleak did not get

The kmemleak_seq_next() function tries to get an object (and increment
its use count) before returning it. If it could not get the last object
during list traversal (because it may have been freed), the function
should return NULL rather than a pointer to such object that it did not
get.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Acked-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Cc: <stable@kernel.org>