selftests/bpf: Fix warning comparing pointer to 0

Avoid pointer type value compared with 0 to make code clear. Reported by
coccicheck:

  tools/testing/selftests/bpf/progs/map_ptr_kern.c:370:21-22:
  WARNING comparing pointer to 0
  tools/testing/selftests/bpf/progs/map_ptr_kern.c:397:21-22:
  WARNING comparing pointer to 0

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/1648605588-19269-1-git-send-email-baihaowen@meizu.com