wrapper: fix segfault in cgroup_get_uid_gid()

wrapper: fix segfault in cgroup_get_uid_gid()

The arguments passed to cgroup_get_uid_gid() are of type pointers and
the user might pass NULL in place of or all of the arguments, causing
a segfault. segfault is triggered when the NULL, argument value is
passed without check, fix it by checking for NULL before proceeding.

Reproducer:
-----------

int main(void)
{
	struct cgroup *cgrp;
	uid_t tuid, cuid;
	gid_t tgid, cgid;

	int ret;

	ret = cgroup_init();
	if (ret) {
		printf("Failed to initialize: %s\n", cgroup_strerror(ret));
		exit (1);
	}

	cgrp = cgroup_new_cgroup("fuzzer");
	if (!cgrp) {
		printf("Failed to allocate cgroup fuzzer\n");
		exit(1);
	}

	ret = cgroup_create_cgroup(cgrp, 1);
	if (ret) {
		printf("failed to create %s: %s\n", "fuzzer", cgroup_strerror(ret));
		goto err;
	}

	cgroup_get_uid_gid(cgrp, NULL, NULL, NULL, NULL);
	// should not reach here

	return 0;
}

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>