bpf: Fix warning comparing pointer to 0

Fix the following coccicheck warning:

./tools/testing/selftests/bpf/progs/fentry_test.c:67:12-13: 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: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1615360714-30381-1-git-send-email-jiapeng.chong@linux.alibaba.com