LIBZMQ-497 there could be unsent bytes in encoder

When we send a large message, the message can be splitted into two chunks.
One is in the encoder buffer and the other is the zero-copy pointer.
The session could get the term before the last chunk is sent.

src/stream_engine.cpp

@@ -65,6 +65,7 @@ zmq::stream_engine_t::stream_engine_t (fd_t fd_, const options_t &options_, cons
     options (options_),
     endpoint (endpoint_),
     plugged (false),
+    terminating (false),
     socket (NULL)
 {
     //  Put the socket into non-blocking mode.
@@ -188,6 +189,11 @@ void zmq::stream_engine_t::unplug ()
 
 void zmq::stream_engine_t::terminate ()
 {
+    if (!terminating && encoder && encoder->has_data ()) {
+        //  Give io_thread a chance to send in the buffer
+        terminating = true;
+        return;
+    }
     unplug ();
     delete this;
 }
@@ -298,6 +304,8 @@ void zmq::stream_engine_t::out_event ()
     //  this is necessary to prevent losing incomming messages.
     if (nbytes == -1) {
         reset_pollout (handle);
+        if (unlikely (terminating))
+            terminate ();
         return;
     }
 
@@ -309,6 +317,10 @@ void zmq::stream_engine_t::out_event ()
     if (unlikely (handshaking))
         if (outsize == 0)
             reset_pollout (handle);
+
+    if (unlikely (terminating))
+        if (outsize == 0)
+            terminate ();
 }
 
 void zmq::stream_engine_t::activate_out ()