Fix the null reference vulnerability.

[QiuYitai]

Hello,
Our team has recently been conducting research on a null-pointer-dereference (NPD) vulnerability detection tool and used it to scan xl2tpd(the version on the master branch). After a manual review, we have identified some potentially vulnerable code snippets that may lead to null-pointer-dereference bugs.
The NULL Dereference vulnerability happens in int control_finish(), control.c
How the NULL Pointer Dereference happens:

1. When c->lac == NULL
2. NULL dereference of variable c->lac happens at if (c->lac->pass_peer)

int control_finish (struct tunnel *t, struct call *c)
{
    ......
=>  if (c->lac)
    {
        ......
    }
    ......
=>  if (c->lac->pass_peer)
    {
        ......
    }
    ......
 }