phonet/pep: refuse to enable an unbound pipe

This ioctl() implicitly assumed that the socket was already bound to
a valid local socket name, i.e. Phonet object. If the socket was not
bound, two separate problems would occur:

1) We'd send an pipe enablement request with an invalid source object.
2) Later socket calls could BUG on the socket unexpectedly being
   connected yet not bound to a valid object.

Reported-by:  <syzbot+2dc91e7fc3dea88b1e8a@syzkaller.appspotmail.com>
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/phonet/pep.c

@@ -947,6 +947,8 @@ static int pep_ioctl(struct sock *sk, int cmd, unsigned long arg)
 			ret =  -EBUSY;
 		else if (sk->sk_state == TCP_ESTABLISHED)
 			ret = -EISCONN;
+		else if (!pn->pn_sk.sobject)
+			ret = -EADDRNOTAVAIL;
 		else
 			ret = pep_sock_enable(sk, NULL, 0);
 		release_sock(sk);