samples/landlock: Fix possible NULL dereference in parse_path()

samples/landlock: Fix possible NULL dereference in parse_path()

[ Upstream commit 078bf9438a31567e2c0587159ccefde835fb1ced ]

malloc() may return NULL, leading to NULL dereference.  Add a NULL
check.

Fixes: ba84b0bf5a16 ("samples/landlock: Add a sandbox manager example")
Signed-off-by: Zichen Xie <zichenxie0106@gmail.com>
Link: https://lore.kernel.org/r/20241128032955.11711-1-zichenxie0106@gmail.com
[mic: Simplify fix]
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>