ETIMEDOUT is a valid error code here. Fixes intermittent assertion failure on laggy networks

src/stream_engine.cpp

@@ -308,7 +308,8 @@ int zmq::stream_engine_t::write (const void *data_, size_t size_)
         return 0;
 
     //  Signalise peer failure.
-    if (nbytes == -1 && (errno == ECONNRESET || errno == EPIPE))
+    if (nbytes == -1 && (errno == ECONNRESET || errno == EPIPE ||
+          errno == ETIMEDOUT))
         return -1;
 
     errno_assert (nbytes != -1);