selftests/bpf: Fix warning comparing pointer to 0

Fix the following coccicheck warnings:

  ./tools/testing/selftests/bpf/progs/fexit_test.c:77:15-16: WARNING
  comparing pointer to 0.

  ./tools/testing/selftests/bpf/progs/fexit_test.c:68:12-13: WARNING
  comparing pointer to 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1615881577-3493-1-git-send-email-jiapeng.chong@linux.alibaba.com