wrapper: fix segfault in cgroup_set_value_string()

wrapper: fix segfault in cgroup_set_value_string()

The second and third arguments passed to cgroup_set_value_string() are
of type char * and the user might pass NULL in place of one or both of
the arguments, causing a segfault.  segfault is trigger when the NULL,
argument values are used without checks, fix it by checking for NULL
before proceeding.

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

int main(void)
{
        struct cgroup_controller *cgc;
        struct cgroup *cgrp;

        cgroup_init();
        if (ret)
		exit(1);

        cgrp = cgroup_new_cgroup("fuzzer");
        if (!cgrp)
                exit(1);

        cgc = cgroup_add_controller(cgrp, "cpu");
        if (!cgc)
		exit(1);

        ret = cgroup_create_cgroup(cgrp, 1);
        if (ret)
		exit(1);

        cgroup_set_value_string(cgc, 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>