Problem: Out of memory condition not always checked

Solution: Use the appropriate assertion macro.

src/req.cpp

@@ -84,6 +84,8 @@ int zmq::req_t::xsend (msg_t *msg_)
 
             //  Copy request id before sending (see issue #1695 for details).
             uint32_t *request_id_copy = (uint32_t *) malloc (sizeof (uint32_t));
+            zmq_assert (request_id_copy);
+
             *request_id_copy = request_id;
 
             msg_t id;