Safety measure in zmq_msg_close implemented

zmq_msg_close now empties the message on zmq_msg_close, thus not
leaving random data in the structure, that may be mistaken for
a valid message.
Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

src/zmq.cpp

@@ -154,9 +154,8 @@ int zmq_msg_init_data (zmq_msg_t *msg_, void *data_, size_t size_,
 int zmq_msg_close (zmq_msg_t *msg_)
 {
     //  For VSMs and delimiters there are no resources to free.
-    if (msg_->content == (zmq::msg_content_t*) ZMQ_DELIMITER ||
-          msg_->content == (zmq::msg_content_t*) ZMQ_VSM)
-        return 0;
+    if (msg_->content != (zmq::msg_content_t*) ZMQ_DELIMITER &&
+          msg_->content != (zmq::msg_content_t*) ZMQ_VSM) {
 
         //  If the content is not shared, or if it is shared and the reference.
         //  count has dropped to zero, deallocate it.
@@ -171,6 +170,13 @@ int zmq_msg_close (zmq_msg_t *msg_)
                 content->ffn (content->data, content->hint);
             free (content);
         }
+    }
+
+    //  As a safety measure, let's make the deallocated message look like
+    //  an empty message.
+    msg_->content = (zmq::msg_content_t*) ZMQ_VSM;
+    msg_->flags = 0;
+    msg_->vsm_size = 0;
 
     return 0;
 }