pipe: fix bug in rollback() method

The msgs_written variable keeps track how many complete
messages have been written so far. The rollback operation drops all
fragments of the last incomplete message so it shouldn't
change this variable at all.

src/pipe.cpp

@@ -176,7 +176,6 @@ void zmq::writer_t::rollback ()
     while (pipe->unwrite (&msg)) {
         zmq_assert (msg.flags & ZMQ_MSG_MORE);
         zmq_msg_close (&msg);
-        msgs_written--;
     }
 
     if (stalled && endpoint != NULL && !pipe_full()) {