selftests/mm: fix WARNING comparing pointer to 0

Remove comparing pointer to 0 to avoid this warning from coccinelle:

./tools/testing/selftests/mm/map_populate.c:80:16-17: WARNING comparing pointer to 0, suggest !E
./tools/testing/selftests/mm/map_populate.c:80:16-17: WARNING comparing pointer to 0

Link: https://lkml.kernel.org/r/20230817160033.90079-1-tuananhlfc@gmail.com


Signed-off-by: Anh Tuan Phan <tuananhlfc@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>