UAF handling PGM dataloss event

[i-garrison]

If pgm_recvmsgv returns PGM_IO_STATUS_RESET, the following code stores a pointer into block which is released via pgm_free_skb() call before returning to caller:

       //  Data loss.
        if (status == PGM_IO_STATUS_RESET) {
            struct pgm_sk_buff_t *skb = pgm_msgv[0].msgv_skb[0];

            //  Save lost data TSI.
>>>         *tsi_ = &skb->tsi;
            nbytes_rec = 0;

            //  In case of dala loss -1 is returned.
            errno = EINVAL;
>>>         pgm_free_skb (skb);
            return -1;
        }

https://github.com/zeromq/libzmq/blob/master/src/pgm_socket.cpp#L576-L581

This leads to some funny behavior under load at call site where zmq::pgm_receiver_t::in_event() does peer lookup here
https://github.com/zeromq/libzmq/blob/master/src/pgm_receiver.cpp#L160-L174