ZMQII-43: Race condition when writing to pipe

src/session.cpp

@@ -78,7 +78,7 @@ bool zmq::session_t::read (::zmq_msg_t *msg_)
 
 bool zmq::session_t::write (::zmq_msg_t *msg_)
 {
-    if (out_pipe->write (msg_)) {
+    if (out_pipe && out_pipe->write (msg_)) {
         zmq_msg_init (msg_);
         return true;
     }