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

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

Fixes: ba84b0b ("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>

Author: Gax-c

samples/landlock/sandboxer.c

@@ -91,6 +91,9 @@ static int parse_path(char *env_path, const char ***const path_list)
 		}
 	}
 	*path_list = malloc(num_paths * sizeof(**path_list));
+	if (!*path_list)
+		return -1;
+
 	for (i = 0; i < num_paths; i++)
 		(*path_list)[i] = strsep(&env_path, ENV_DELIMITER);
 
@@ -127,6 +130,10 @@ static int populate_ruleset_fs(const char *const env_var, const int ruleset_fd,
 	env_path_name = strdup(env_path_name);
 	unsetenv(env_var);
 	num_paths = parse_path(env_path_name, &path_list);
+	if (num_paths < 0) {
+		fprintf(stderr, "Failed to allocate memory\n");
+		goto out_free_name;
+	}
 	if (num_paths == 1 && path_list[0][0] == '\0') {
 		/*
 		 * Allows to not use all possible restrictions (e.g. use