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

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

Fixes: ba84b0bf ("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>